feat(pty): add platform-neutral session contract

[decode2]

Linked issue

Closes #33

• 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 a platform-neutral PTY session contract for identity, I/O, resize, and lifecycle operations.
• Adapts the existing Windows ConPTY session to the contract without changing its external behavior.
• Adds contract tests for attribution, backpressure/error boundaries, natural exit, and idempotent close.

Changes

Area or file	Change
crates/splice-pty/src/lib.rs	Defines the shared session contract and implements it for the existing Windows ConPTY session.
crates/splice-pty/tests/session_contract.rs	Exercises the platform-neutral behavioral contract with a deterministic test session.

Test plan

• Ran the focused automated checks: cargo fmt --all --check; cargo test -p splice-pty (16/16 passed); cargo clippy -p splice-pty --all-targets -- -D warnings (passed).
• Exercised the affected terminal, AI CLI, or desktop workflow: contract tests cover identity/early output, input error and acknowledgment backpressure, natural exit, and idempotent close (3/3 passed).
• Verified relevant Windows, Linux, or WSL behavior, or documented why it is not applicable: Windows GNU cross-target/contract and differential Wine evidence passed during SDD verification; Wine cannot validate the native ConPTY runtime, so native Windows CI is required before merge. Unix and WSL backends are outside this PR.

Chained pull request context

• Previous PR: feat(compat): add finite AI CLI compatibility harness #31, merged into master at 1c02999a
• Next PR: PR3 (planned Unix PTY backend), depends on this session contract after PR2 merges
• Review this PR first: the contract semantics in lib.rs, then the contract scenarios in session_contract.rs
• Intentionally out of scope: Unix backend implementation and all later desktop/platform, WSL, workspace, UI, packaging, and documentation slices

Chain Context

Field	Value
Chain	Terminal AI platform expansion
Tracker PR	Not needed (stacked-to-main)
Position	PR2 of 15
Base	master at PR1 merge 1c02999a
Depends on	#31 (merged)
Follow-up	PR3: Unix PTY backend, after this PR merges
Review budget	226 changed lines / 400
Starts at	PR1 compatibility foundation merged into master
Ends with	Platform-neutral PTY session contract backed by preserved Windows ConPTY behavior

Chain Overview

master
  └── ✅ PR1 #31: AI CLI compatibility harness (merged)
       └── 📍 PR2: platform-neutral PTY session contract
            └── PR3: Unix PTY backend (depends on PR2)

Scope

• Includes: shared PTY session contract, Windows ConPTY adaptation, and contract tests.
• Excludes: Unix backend implementation and every later chain slice.

Autonomy

• CI is expected to pass for this PR branch; native Windows CI remains a required pre-merge check.
• This PR has one deliverable scope.
• This PR can be rolled back without unrelated changes.
• Tests and verification cover this unit.

Rollback

Revert commit cf7306b to remove only the shared session contract, its Windows adaptation, and session_contract.rs; PR1 and unrelated PTY flow-control behavior remain intact.

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.
• I updated documentation when behavior or compatibility changed (not applicable: this internal contract preserves current user-visible behavior).
• My commits follow Conventional Commits.
• My commits contain no AI attribution, including Co-Authored-By trailers.