refactor!: rename Contract.root/exp_date to symbol/expiration (closes #467)

[userFRM]

Summary

Closes #467.

Rename the public-API fields on the SDK to match the v3 vendor surface
documented in the v2 → v3 migration guide:

Contract.root -> Contract.symbol
Contract.exp_date -> Contract.expiration
OptionContract.root -> OptionContract.symbol
FlatFileRow.root -> FlatFileRow.symbol
IndexEntry.root -> IndexEntry.symbol
IndexEntry.exp -> IndexEntry.expiration

Plus the matching constructor + parameter renames (Contract::stock(symbol),
Contract::option(symbol, expiration, ...), Contract::option_raw(...),
FlatFileRow::from_decoded(symbol, expiration, ...)).

Bumps the workspace 8.0.26 → 8.0.27 and tdbe 0.12.7 → 0.12.8 (the latter
because crates/tdbe/src/types/tick_generated.rs::OptionContract.symbol
is regenerated from the schema rename).

The wire format is unchanged. Contract::to_bytes / Contract::from_bytes
still serialize the field as root per Contract.java parity, and the
FLATFILES decoder still resolves both v2 (root) and v3 (symbol)
response columns through the existing decode::HEADER_ALIASES alias.

Per-language renames

Rust

• thetadatadx::fpss::protocol::Contract:
  • .root → .symbol, .exp_date → .expiration
  • Contract::stock(root) → Contract::stock(symbol)
  • Contract::index(root) → Contract::index(symbol)
  • Contract::rate(root) → Contract::rate(symbol)
  • Contract::option(root, exp_date, …) → Contract::option(symbol, expiration, …)
  • Contract::option_raw(root, exp_date, …) → Contract::option_raw(symbol, expiration, …)
• tdbe::types::tick::OptionContract.root → OptionContract.symbol
• thetadatadx::flatfiles::FlatFileRow.root → FlatFileRow.symbol

Python (thetadatadx)

• contract.root / contract.exp_date → contract.symbol / contract.expiration
• OptionContract(root=…) constructor keyword → OptionContract(symbol=…)

TypeScript (@thetadatadx/napi)

• contract.root / contract.expDate → contract.symbol / contract.expiration
• OptionContract.root → OptionContract.symbol

Go (thetadatadx)

• c.Root / c.ExpDate → c.Symbol / c.Expiration
• OptionContract.Root → OptionContract.Symbol

C++ (tdx::OptionContract, TdxContract, TdxOptionContract)

• c.root / c.exp_date / c.has_exp_date → c.symbol / c.expiration / c.has_expiration
• OptionContract.root → OptionContract.symbol

C ABI

• TdxContract.root → TdxContract.symbol
• TdxContract.exp_date → TdxContract.expiration
• TdxContract.has_exp_date → TdxContract.has_expiration
• TdxOptionContract.root → TdxOptionContract.symbol

FLATFILES (CSV / JSONL)

• Option / index contract-prefix headers: root,expiration,strike,right,… → symbol,expiration,strike,right,…
• Stock contract-prefix headers: root,… → symbol,…
• JSONL object keys mirror the headers ("symbol" instead of "root").

REST / WebSocket / MCP outputs

• tools/server REST + WS JSON outputs emit "symbol" and "expiration" keys on every contract payload.
• tools/mcp option_list_contracts rows emit "symbol" instead of "root".
• WS subscribe envelope accepts {"contract": {"symbol": "…"}} and falls back to legacy {"root": "…"} for backwards compatibility.

SSOT touch points

• crates/thetadatadx/tick_schema.toml: OptionContract column field renamed root → symbol.
• crates/thetadatadx/build_support/fpss_events/{python,typescript,go_structs,ffi_rust,ffi_c}.rs: emit Contract.symbol / Contract.expiration on every binding plus the regenerated FFI converter stages a contract_symbol_cstring.
• crates/thetadatadx/build_support/ticks/go.rs: updates the OptionContract source-expression dispatch.
• crates/thetadatadx/build_support/ticks/rust_frames.rs + crates/thetadatadx/build_support/ticks/cli_headers.rs: Arrow / Polars / CLI raw-header column names track column.field instead of column.name, so the divergence between wire root and public symbol flows through automatically.
• crates/thetadatadx/build_support/sdk_surface/templates/go/next_event_body.go.tmpl: reads c.symbol / c.expiration off the C ABI struct and assigns Symbol / Expiration on the Go-side *Contract.
• crates/thetadatadx/build_support/endpoints/render/templates/cpp/option_contracts_convert.cpp.tmpl: assigns c.symbol on the C++ OptionContract value type.

Hand-written touch points

• crates/thetadatadx/src/fpss/protocol.rs — Contract.symbol, Contract.expiration; constructor + method param renames; the byte-level decoder keeps the wire-spec local names (root, exp_date) so the file still diffs cleanly against the upstream binary protocol, but the struct construction now binds to the v3 names.
• crates/thetadatadx/src/decode.rs — parse_option_contracts_v3 constructs OptionContract { symbol, … }; the alias ("root", "symbol") continues to map v3 wire columns onto the find_header("root") lookup.
• crates/thetadatadx/src/flatfiles/{decoded.rs,decoded_row.rs,index.rs,writer.rs} — symbol/expiration field renames + CSV / JSONL header rename.
• crates/thetadatadx/src/{fpss,unified}.rs — every contract.root / contract.exp_date access updated.
• ffi/src/{streaming.rs,types.rs} — TdxOptionContract.symbol, Contract::symbol accessors.
• tools/{cli,mcp,server} — option-contract rendering reads .symbol, REST + MCP + WS JSON outputs emit "symbol" / "expiration" keys on every contract payload.
• sdks/cpp/include/{thetadx.h,thetadx.hpp} — TdxOptionContract.symbol, OptionContract.symbol.
• sdks/go/tick_ffi_mirrors.go — cOptionContract.Symbol.
• sdks/typescript/index.d.ts — Contract.symbol / Contract.expiration, OptionContract.symbol.

Regenerated bindings

• ffi/src/{fpss_event_structs,fpss_event_converter}.rs
• sdks/python/src/{fpss_event_classes,tick_classes,tick_arrow}.rs
• sdks/typescript/src/{fpss_event_classes,tick_classes}.rs
• sdks/go/{fpss_event_structs,fpss_event_structs.h.inc,fpss_methods,tick_converters,tick_structs}.go
• sdks/cpp/include/fpss_event_structs.h.inc, sdks/cpp/src/historical.cpp.inc
• crates/thetadatadx/src/frames_generated.rs
• tools/cli/src/raw_headers_generated.rs

CHANGELOG / docs

• CHANGELOG.md and docs-site/docs/changelog.md get a [8.0.27] - 2026-05-06 entry under ### Breaking with the per-language rename matrix and a ### Changed bullet noting the tdbe bump.
• docs-site/docs/{getting-started,streaming,historical/option/list}/*.md examples updated to use contract.symbol / contract.expiration.

Test plan

Local CI gate (all green on the final push):

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo deny check
• cargo run -p thetadatadx --bin generate_sdk_surfaces --features config-file -- --check
• cargo check / clippy / test --manifest-path tools/server/Cargo.toml
• cargo check / clippy / test --manifest-path tools/mcp/Cargo.toml
• cargo check --manifest-path sdks/python/Cargo.toml
• cargo check --manifest-path sdks/typescript/Cargo.toml

Closes #467