[mikkel]

Closes gap #3 from the JS↔Python parity audit. When Python's smartapply refuses a model rewrite it raises SmartApplyError. JS threw a plain Error, so callers couldn't tell an intent refusal apart from a provider or network failure.

This is a behavior change for refusals

When smartapply refuses a model rewrite, the object it throws changes:

	before	after
class / constructor	Error	SmartApplyError (err.constructor === SmartApplyError; still instanceof Error)
err.name	'Error'	'SmartApplyError'
printed form (String(err), first line of err.stack, uncaught output)	Error: smartapply: refusing to write …	SmartApplyError: smartapply: refusing to write …

Code that checks err.name === 'Error' or err.constructor === Error, or that snapshots the printed error, will see the difference.

What did not change:

• The message text: smartapply: refusing to write "<path>": the model changed lines the diff did not add or remove. err.message checks and /did not add or remove/ matchers keep working.
• When a refusal happens: the same condition, at the one call site.
• Provider and LLM errors: they still propagate as the original error object. They're never wrapped, and they're never a SmartApplyError.

CHANGELOG.md (0.3.1, unreleased) now records this under "Behavior changes", and its summary no longer says smartapply is unchanged.

Changes

• src/gptdiff.js: adds export class SmartApplyError extends Error, which sets name = 'SmartApplyError' and has the constructor (message), like Python's. It's thrown at the refusal site, and the smartapply JSDoc gains a @throws note.
• src/index.js: exports SmartApplyError. package.json exports is unchanged.
• .d.ts: regenerated with npm run types (tsc 5.9.3 from the lockfile). This also brings main's out-of-date src/filter.d.ts up to date (SNAKE_CASE_OPTIONS and the filterRelevant @throws note from filterRelevant: reject Python-style snake_case options #7). That part is generated type output only.
• CHANGELOG.md: the 0.3.1 entry described above.
• No landing/lib change, so no site redeploy is needed. landing/lib only holds copies of env.js, filter.js, llm.js and text.js.

Tests (test/smartapply_error.test.js, 6 tests, all LLM calls mocked)

• The export: same class as the one in src/gptdiff.js, instanceof Error, name, and String(err).
• A refused rewrite rejects with instanceof SmartApplyError and the exact previous message, and the input isn't mutated.
• An empty rewrite is refused with SmartApplyError.
• A provider error (status: 402) from the injected callLlmForApply comes through as the same object, not a SmartApplyError.
• A TypeError('fetch failed') from callLlm passes through callLlmForApply unchanged.
• The deterministic fast path still doesn't call the model.

Local run

npm test on Node 20.19.2 at the head: 238 tests: 234 pass, 0 fail, 4 skipped. The skips are the live tests, which only run with LIVE_LLM / GPTDIFF_LIVE, so no LLM calls were made.

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