perf(runtime-tags): skip serializing unused controllable <let> change handler

[DylanPiercey]

A controllable <let>'s valueChange handler (TagVariableChange:<binding>) is only read back on resume by _let_change, and only when the variable is actually reassigned on the client. Reactive value= updates run in rendering mode and re-set the handler from the fresh argument, so they don't depend on the serialized copy — only a direct assignment (the signal's entry point) does.

The handler is now serialized only when the binding has assignment sites. The decision is made in finalizeReferences over the marked controllable-<let> bindings (rather than the post-prune bindings set), so it still applies when the <let>'s value was pruned but it's still assigned + has a change handler.

No behavior change: only writes.*.html / html.bundle.* / sizes.json snapshots changed — no render/dom.bundle diffs. Two fixtures with a never-reassigned controllable <let> (let-bind-to-undefined, let-bind-stateful-upstream-alias) drop the dead handler from their resume payload.

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Generated by Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: a731a01

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
marko	Patch
@marko/runtime-tags	Patch

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 706006b1-9076-4285-9f94-7c01eb1522b1

📥 Commits

Reviewing files that changed from the base of the PR and between 0048472 and a731a01.

⛔ Files ignored due to path filters (8)

• packages/runtime-tags/src/__tests__/fixtures/let-bind-stateful-upstream-alias/__snapshots__/html.bundle.debug.js is excluded by !**/__snapshots__/** and included by **
• packages/runtime-tags/src/__tests__/fixtures/let-bind-stateful-upstream-alias/__snapshots__/html.bundle.js is excluded by !**/__snapshots__/** and included by **
• packages/runtime-tags/src/__tests__/fixtures/let-bind-stateful-upstream-alias/__snapshots__/writes.debug.html is excluded by !**/__snapshots__/** and included by **
• packages/runtime-tags/src/__tests__/fixtures/let-bind-stateful-upstream-alias/__snapshots__/writes.html is excluded by !**/__snapshots__/** and included by **
• packages/runtime-tags/src/__tests__/fixtures/let-bind-to-undefined/__snapshots__/html.bundle.debug.js is excluded by !**/__snapshots__/** and included by **
• packages/runtime-tags/src/__tests__/fixtures/let-bind-to-undefined/__snapshots__/html.bundle.js is excluded by !**/__snapshots__/** and included by **
• packages/runtime-tags/src/__tests__/fixtures/let-bind-to-undefined/__snapshots__/writes.debug.html is excluded by !**/__snapshots__/** and included by **
• packages/runtime-tags/src/__tests__/fixtures/let-bind-to-undefined/__snapshots__/writes.html is excluded by !**/__snapshots__/** and included by **

📒 Files selected for processing (5)

• .changeset/let-change-handler-serialization.md
• packages/runtime-tags/src/__tests__/fixtures/let-bind-stateful-upstream-alias/sizes.json
• packages/runtime-tags/src/__tests__/fixtures/let-bind-to-undefined/sizes.json
• packages/runtime-tags/src/translator/core/let.ts
• packages/runtime-tags/src/translator/util/references.ts

Walkthrough

The PR changes runtime-tags so controllable <let> bindings are tracked when valueChange is present, and finalizeReferences only adds TagVariableChange serialization for tracked bindings that have assignmentSections. It also updates two size fixtures and marks marko and @marko/runtime-tags for patch releases in the changeset.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: skipping serialization of an unused controllable <let> change handler.
Description check	✅ Passed	The description accurately explains the runtime-tags serialization change and its snapshot-only impact.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/runtime-tags-serialization-uuq9w5

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.25%. Comparing base (f368176) to head (a731a01).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3293   +/-   ##
=======================================
  Coverage   94.25%   94.25%           
=======================================
  Files         383      383           
  Lines       50869    50894   +25     
  Branches     3855     3859    +4     
=======================================
+ Hits        47946    47971   +25     
  Misses       2897     2897           
  Partials       26       26           

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