pxf: Result::Directives() / Result::Tables() accessors

[trendvidia]

Summary

Third PR in the v0.72–v0.75 cpp catch-up. The fast-path decoder previously walked directives just enough to satisfy the standalone constraint and arity checks; it discarded their content. This PR wires the parsed shape onto `Result` so consumers can read the document-root directive list after `UnmarshalFull` returns.

API additions on `Result`:

• `Directives()` → `const vector&` — generic `@ *(prefix) [{ ... }]` blocks in source order. `body` holds the raw bytes between `{` and `}`, preserved verbatim for downstream re-parsing (chameleon's `@header` reader, etc.). Single-prefix populates the back-compat `type` field per v0.72.0 shape.
• `Tables()` → `const vector&` — `@table` directives with full column metadata and parsed cell `ValuePtr` per row, faithful to the three-state cell grammar (absent / present-but-null / present-with-value).

Fast path (`decode_fast.cc`):

• `ConsumeDirectives` now builds `Directive` / `TableDirective` structs inline, conditionally appending to `result_` when non-null
• `Unmarshal` (`Result=nullptr`) retains its zero-allocation contract: the fast path still walks and validates directives but allocates nothing on the prelude
• New `ParseScalarCellValue` helper mirrors the scalar branches of the AST parser's `ParseValue`. Used by `@table` row parsing; list / block cell tokens are already rejected before it's called
• Body bytes for `@` blocks are sliced via `lex_.Input().substr(open + 1, close - (open + 1))` using the `Position::offset` added in PR pxf: parser-side @<name>/@entry/@table directive grammar #9

Test plan

• 14 new tests in `pxf_result_directives_test.cc` covering bare/single/multi-prefix directives, nested block body preservation, source-order multi-directive, `@type` not leaking into `Directives()`, `@table` columns / rows / concrete cell values, three-state cells, multi-table, directive+table coexistence, and the `Unmarshal` (no Result) regression check
• All 198 tests pass locally (184 on main → +14 new)
• CI green (clang-format, all matrix jobs, CodeQL)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 51.93798% with 62 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pxf/decode_fast.cc	50.40%	50 Missing and 12 partials ⚠️

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