[codex] prepare 0.10.0 release

[lbliii]

Summary

Prepare Kida 0.10.0 as a focused diagnostics release.

• bump package metadata and lockfile to 0.10.0
• add 0.10.0 changelog and release-note collateral
• add the release roadmap with steward synthesis, parity matrix, not-now scope, and verification record
• fix diagnostics docs examples for component-stack tuple order and ErrorCode.UNDEFINED_VARIABLE
• update README and installation docs for 0.10.0
• preserve template-enhanced diagnostics for async streaming errors while keeping the original exception in __cause__

Steward Notes

• Consulted root AGENTS.md, plan/AGENTS.md, site/AGENTS.md, tests/AGENTS.md, and src/kida/template/AGENTS.md.
• Accepted scope: diagnostics hardening, source attribution, async streaming parity, and release collateral.
• Deferred scope: new public diagnostic dataclass exports, CLI/API knobs, schema changes, workflow changes, sandbox semantics, and benchmark-baseline churn.
• Remote-state note: remote main already contains the structured diagnostics work as squash commit 66cf8ec from PR [codex] Improve UndefinedError diagnostics for framework views #132, so this branch is based on current origin/main and avoids replaying the local pre-squash commit history.

Validation

• uv run pytest tests/test_diagnostics_contract.py tests/test_public_api_snapshot.py tests/test_kida_async_rendering.py::TestAsyncExceptionPropagation::test_error_in_async_iterable_propagates
• make docs
• make verify-stability
• Coverage summary from the final gate: 84.1% against the 83% floor.

Release Follow-Up

Publishing has not been performed. After this PR is merged, the remaining release operations are the remote v0.10.0 tag/GitHub release, PyPI publish via the release workflow, and the floating v0 action tag update.

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