test(platform): add native runtime matrix

[decode2]

Linked issue

Closes #46

• 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 an authoritative native Windows ConPTY gate and a pinned Ubuntu 24.04 PTY runtime lane.
• Adds deterministic runtime and CI-contract fixtures with explicit unsupported-host skips, without adding or claiming WSL support.

Changes

Area or file	Change
.github/workflows/ci.yml	Names the Windows ConPTY authority gate and adds the Ubuntu 24.04 native runtime receipt lane.
crates/splice-pty/tests/platform_runtime_matrix.rs	Verifies runtime support boundaries, live PTY behavior, and workflow contracts.

Test plan

• Ran focused and relevant automated checks: runtime matrix 3/3 (two explicit Ubuntu 26.04 skips), splice-pty 34/34, platform authority 2/2, frontend 203/203; exit 0.
• Exercised the affected native Unix PTY workflow: startup, input, resize, interrupt, liveness, teardown, close, identity, and backpressure passed on the local Linux host.
• Verified platform boundaries: Windows GNU test cross-compilation passed; native Windows ConPTY and Ubuntu 24.04 execution require this PR's CI receipts. WSL2/WSLg is intentionally not tested or claimed in this slice.
• Quality gates passed: Clippy with -D warnings, Rust formatting, ESLint, actionlint v1.7.10, git diff --check, pre-commit, pre-push, and pre-PR.

Verification receipts and warnings

• Review authority: approved review-528636fec7918e68; candidate tree e4a055d0beaf530dac6de2dbf0a1a33e320640c7.
• SDD verification: PASS WITH WARNINGS, 4/4 scoped requirements and 4/4 scenarios, no blockers or critical findings.
• Native receipts required before support claims: Windows ConPTY gate and Ubuntu 24.04 runtime lane must pass in CI.
• Accepted non-blocking warnings: unreadable /etc/os-release can skip; sequential Ubuntu checks reduce failure receipts; ConPTY runs explicitly and in workspace tests; Ubuntu 22.04 is admitted by the fixture but not selected by this CI slice.
• No WSL workflow, runtime receipt, or support claim is included; WSL is deferred to PR9/task 3.3b.

Chained pull request context

• Previous PR: feat(platform): wire target-aware PTY sessions #45
• Next PR: PR9/task 3.3b, planned WSL2/WSLg runtime matrix
• Review this PR first: .github/workflows/ci.yml platform gate authority, then platform_runtime_matrix.rs support/skip contracts
• Intentionally out of scope: WSL2/WSLg, workspaces, UI, packaging, and documentation

Field	Value
Chain	Terminal AI platform expansion
Tracker PR	Not needed
Position	PR8 of 16
Base	master
Depends on	#45
Follow-up	PR9/task 3.3b WSL2/WSLg runtime matrix
Review budget	160 / 400 changed lines
Starts at	be9cbc7 / merged PR #45
Ends with	Native Windows and Ubuntu runtime support gates with deterministic contracts

Chain overview

master
  └── #45 Platform client wiring
       └── 📍 PR8 Native runtime matrix
            └── PR9 WSL2/WSLg runtime matrix (planned)

Scope

• Includes: native Windows ConPTY and Ubuntu 24.04 runtime gates and contract tests.
• Excludes: WSL2/WSLg and all later platform-expansion work units.

Autonomy

• CI is expected to pass for this PR branch
• This PR has one deliverable scope
• This PR can be rolled back without unrelated changes
• Tests and verification cover this unit

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 intentionally out of scope; this test/CI slice makes no new support claim before native receipts pass.
• My commits follow Conventional Commits.
• My commits contain no AI attribution, including Co-Authored-By trailers.