Autodiff coverage: close silent dispatch gaps + KSP-generated coverage guard

[michalharakal]

Closes #773.

Differentiability is a three-layer contract that must agree per op: @Diff (contract) → a generated DifferentiableTensorOps.<rule>Backward (formula) → a dispatch arm in buildBackwardFromTrace (wiring). KSP already made a missing formula a compile error, but the wiring was hand-maintained in a downstream module and unguarded — so elu, leakyRelu and permute had correct backward formulas that were never dispatched, and their gradients were silently dropped to null.

Close the gaps

• Wire elu / leakyRelu / permute into the dispatch. Fix permuteBackward's attribute decode (the tracer records axes as List<Int>, not IntArray — it would have thrown the first time it ran, confirming it never had).
• Make cos / sin / tril / gather / indexSelect / unfold / convTranspose1d differentiable: @Diff + a backward formula + a finite-difference parity test each. The structural ones are raw-loop adjoints in DefaultExecutionTape (like conv1dBackward) — no new forward primitives, no StableHLO converter changes. sin/cos/convTranspose1d become abstract on TensorOps so the tracing wrapper records them (both concrete backends already implement them).
• Document sign / ge / lt / convert as non-differentiable by design.

Make the drift impossible (the core change)

• TracingWrapperProcessor now also emits DifferentiableTensorOpsRules.ruleNames — the authoritative set of @Diff rule names.
• The dispatch when is refactored into a name-keyed backwardDispatch map exposing dispatchedOpNames.
• New AutodiffCoverageTest asserts ruleNames ⊆ dispatchedOpNames — failing CI the instant any @Diff op lacks a wired backward (it would have caught the elu/leakyRelu/permute bug).
• OperatorDocProcessor now emits isDifferentiable (+ optional diffRuleName) per function in operators.json, with the schema extended and validateOperatorSchema keeping it well-formed — the manifest becomes the single source of differentiability truth.

Tests

• AutodiffCoverageTest (dispatch covers every @Diff op).
• OpsAutodiffBackwardTest — finite-diff parity for all 10 newly-correct ops.
• Full jvmTest green for skainet-lang-core, skainet-compile-dag, skainet-compile-core, skainet-backend-cpu, skainet-compile-hlo; validateOperatorSchema green.

Considered & rejected

Generating the dispatch table itself from @Diff (zero hand-wiring): blocked by @Diff SOURCE retention + the tape living in a downstream module (KSP in skainet-lang-core can't see those annotations from, or emit into, skainet-compile-dag). The generated rule-set + coverage test gives the same no-drift guarantee today.

🤖 Generated with Claude Code

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• Operator documentation: docs/modules/operators/_generated_/
• JSON schema output: operators.json

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