feat(pty): add Unix PTY spawn and I/O

[decode2]

Linked issue

Closes #35

• The linked issue has the status:approved label.

Pull request type

Select exactly one option and apply the matching type:* label.

• Bug fix (type:bug)
• New feature (type:feature)
• Documentation (type:docs)
• Refactor (type:refactor)
• Maintenance or tooling (type:chore)
• Breaking change (type:breaking-change)

Summary

• Adds the Unix portable-pty backend for structured spawn, shell startup, buffered UTF-8 output, input, process identity, and basic liveness.
• Adds bounded root-process close escalation and live PTY resize safety while keeping process-group descendant teardown and remaining review warnings for PR4.
• Preserves the approved four-path candidate ca1475ad489814327255cb4148584369825ad524 from lineage review-bccf9b5e0bf02b61.

Changes

Area or file	Change
crates/splice-pty/src/lib.rs	Implements Unix PTY spawn, I/O, interrupt, root close, resize, and session lifecycle behavior.
crates/splice-pty/tests/unix_pty.rs	Covers structured argv, cwd/env, UTF-8 streaming, shell I/O, interrupt, bounded close/drop, and resize.
crates/splice-pty/Cargo.toml	Adds Unix-only PTY/runtime dependencies.
Cargo.lock	Records generated dependency resolution.

Review budget

• Authored: 400 lines (373 additions + 27 deletions), excluding generated Cargo.lock.
• Generated: 104 lines in Cargo.lock (102 additions + 2 deletions).
• Complete PR diff: 504 lines across exactly four reviewed paths.
• Review boundary: Unix PTY spawn/basic I/O plus bounded root close/resize safety.

Test plan

• Ran focused automated checks: cargo fmt --all -- --check passed; cargo test -p splice-pty passed 25/25 native tests; cargo clippy -p splice-pty --all-targets -- -D warnings passed.
• Exercised the affected terminal workflow: live Unix PTY tests cover structured spawn, shell input/output, ETX interrupt, malformed/split UTF-8, bounded close/drop, and terminal resize.
• Verified relevant platform behavior: native Linux checks pass. Wine is not a native ConPTY authority and prior Wine probes showed environment-sensitive command-output and explicit-close timing; native Windows CI is required before merge.

Chained pull request context

Strategy: stacked-to-main, PR3 of 15. Each slice targets master after its dependencies merge.

PR1  merged ✅
  ↓
PR2  merged ✅ (`master` at 23af77a)
  ↓
PR3  📍 Unix PTY spawn/basic I/O + bounded root close/resize
  ↓
PR4  planned; depends on PR3 and owns process-group descendants + deferred warnings
  ↓
PR5-15 planned

• Previous PR: PR2, merged into master at 23af77a.
• Next PR: PR4, process-group descendant lifecycle and deferred bounded-review warnings.
• Review this PR first: the four-path Unix PTY work unit and its live Unix tests.
• Intentionally out of scope: process-group descendant teardown, deferred warning IDs, native Windows implementation changes, and all later PR5-15 scope.

Rollback

Revert commit f0a2baf6f59c67d04e124d84cb437d438b32878c to remove the Unix backend, its tests, and its dependency/lock changes without affecting merged PR1-2 behavior.

Contributor checklist

• I searched for duplicate issues and pull requests before starting.
• I linked an approved issue using Closes, Fixes, or Resolves.
• I selected exactly one pull request type and applied exactly one matching type:* label.
• I completed every relevant template field and included reproducible test evidence.
• Documentation is not changed because this internal backend slice does not alter the documented public contract.
• My commits follow Conventional Commits.
• My commits contain no AI attribution, including Co-Authored-By trailers.