Enforce per-field schema version availability at parse time

[Gudge (MGudgin)]

This PR adds per-field schema version availability: a wire field can declare the range of config schema versions it is valid in, and the parser rejects any use outside that range. It is what makes shape-only support for older schema versions real — until now such an annotation would have been documentation that nothing honoured.

Replaces #775 (closed), rebased onto the current main tip with three compile breaks fixed.

Details

• New mxc_version_derive proc-macro crate. #[derive(VersionAvailability)] lifts #[mxc_version(since = "0.8")] / until off wire.rs into metadata normal builds carry, so one declaration feeds both the parser and schema generation (published as x-mxc-since / x-mxc-until). A derive rather than #[schemars(extend(...))], which sits behind the schema-gen feature and so can never be consulted by the parser.
• The mechanism would fail open if a derived JSON name ever disagreed with what serde accepts — the range would simply never fire — so that case is guarded twice: the macro compile-errors on every serde construct it cannot model exactly (flatten, split rename/rename_all, unknown rename_all rules, data-carrying variants), and a conformance test cross-checks all 32 wire types against the names schemars derives independently.
• The gate runs immediately after deserialisation in every entry point, because convert_wire_config moves fields out of the config. State-aware requests are gated on the original document, not the experimental-masked copy.
• version is now required — it selects the legal field surface, so an absent one would silently opt out of every range.
• New version_incompatible code across all five surfaces (Rust MxcErrorCode, engine ErrorCode, TS, C#, MXC_STATUS_VERSION_INCOMPATIBLE = 13) carrying details: { field, declaredVersion, since, until }. The supported-range error migrates onto it. This changes an existing error's observable shape — a consumer string-matching the old range message is affected.
• Only three fields are annotated (seatbelt since 0.7, processContainer.captureDenials / learningMode since 0.8). What is deliberately not annotated matters more: schema-first-appearance is only a lower bound on accepted surface, so experimental (an open block before 0.8) and the state-aware discriminators carry no range — annotating them from schema data would reject configs that have always worked. The new check-version-availability.js oracle gate is fail-closed on exactly those surfaces.
• Rebase fixes: main's feat(sdk): add captureDenials to the typed SandboxPolicy #748 added capture_denials to the typed SandboxPolicy, which breaks SandboxPolicy literals that git auto-merges cleanly, so they surface only at compile time. src/core/mxc-sdk/tests/sandbox.rs and two #[cfg(target_os = "macos")] tests in mxc_engine/src/policy.rs did not compile on Enforce per-field schema version availability at parse time #775's head; the macOS pair is invisible to a Windows cargo check.

Tests

• cargo test --workspace — 2031 passed, 0 failed. cargo fmt --all -- --check, cargo check --workspace --all-targets and cargo clippy --workspace --all-targets -- -D warnings all clean.
• cargo check --target aarch64-apple-darwin -p mxc_engine -p wxc_common -p mxc-sdk --all-targets clean — this is what caught the two macOS test breaks that a green Windows run had hidden.
• Feature-gated builds clean: wxc_common {schema-gen, microvm}, mxc_ffi {dotnetsdk}, mxc_engine {isolation_session}, wxc {isolation_session, microvm, tier2_bfs, wslc, hyperlight}.
• Versioning gates: 125 pass / 0 fail. check-version-availability (3 declared ranges agree with the 0.6 / 0.7 / 0.8 schemas), validate-configs (201 configs), check-schema-versions, check-schema-codegen, check-sdk-types-codegen, and check-dev-schema-compat against phase 7d all green.
• Node SDK build + 223 pass / 6 skipped; C# SDK 35 pass; ErrorCode parity 17 codes; C# bindings codegen OK; version sync OK.
• Non-regression: the Bring network wire schema to full GA spec (wire.rs + config fixtures only) #676 replay still yields exactly 6 findings naming allowLocalNetwork, allowedHosts, blockedHosts, defaultPolicy, enforcementMode, proxy. SUPPORTED_VERSION unchanged at >=0.6, <=0.8.
• Not executed on this host (Windows): the macOS Seatbelt runtime paths, the Windows Sandbox and IsolationSession PowerShell lifecycle suites, and the host-gated MicroVM / Hyperlight E2E configs. The macOS code cross-compiles clean but has not been run.

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Stacked on #732 (phase 7d). Replaces #775.

Co-authored-by: Branden Bonaby bbonaby@microsoft.com
Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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