Reconcile Firmament V2 syntax docs and normalize V2 fixture metadata

[yuechen-li-dev]

Motivation

• Broader CLI/fixture corpus validation exposed missing or malformed leading metadata in several Firmament V2 language fixtures which caused corpus tests to fail and needed hygiene fixes.
• Provide a short, authoritative Phase 1 reconciliation for let, Forge-backed concepts, and the role of templates so future work does not invent conflicting grammar.
• Keep this change documentation + fixture metadata hygiene only, avoiding any new language, parser, or Forge runtime behavior.

Description

• Added docs/v2/firmament-v2-concept-template-syntax-reconciliation.md containing Phase 1 doctrine: let semantics, concept-family application shape, template vs concept distinction, parser vs Forge responsibilities, pmi keyword guidance, canonical example, and explicit deferrals.
• Linked the new reconciliation note from the Phase 1 scope contract at docs/v2/aetheris-v2-phase1-scope-contract.md without rewriting the contract text.
• Normalized leading fixture metadata for V2 Language fixtures by adding stable fixture-id, case, category, validity, implementation, expected, and expected-stage where missing in these files: concept-applications-forge.valid.firmfixture, let-arithmetic-expressions.valid.firmfixture, let-primitive-literals.valid.firmfixture, let-record-groups.valid.firmfixture, let-toleranced-values.valid.firmfixture, and pmi-record-shaped-basic.valid.firmfixture.
• Preserved the F1 concept-application purpose (process<CNC>, hole<Countersink>) and set parser-backed/expected-stage metadata (commonly feature-air) rather than changing parser or Forge logic, and treated air-x7-firmfixture-metadata-invalid as the loader diagnostic for malformed/missing leading metadata.

Testing

• Ran dotnet restore Aetheris.slnx which completed successfully.
• Ran dotnet build Aetheris.slnx -f net10.0 --no-restore /m:1 which succeeded (pre-existing compiler warnings only).
• Ran dotnet test Aetheris.CLI.Tests/Aetheris.CLI.Tests.csproj -f net10.0 --filter "Help|Cli|ExitCode" which passed (275 tests).
• Ran dotnet test Aetheris.Kernel.Firmament.Tests/Aetheris.Kernel.Firmament.Tests.csproj -f net10.0 --filter "Fixture|Corpus|Forge|Concept|FirmamentV2|Language" which passed (197 tests).

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Codex Task