ci: Check bundle size increases per PR

[nicohrubec]

• Base-branch CI measures bundle sizes and saves them as baseline artifacts.
• On each PR, Size Check first checks for Accept Bundlesize Increase. If present, it passes without measuring again.
• Otherwise, it measures the PR bundles, compares them with the target branch's saved baseline, and updates the size-report table.
• If any existing gzipped bundle grows by more than 500 bytes, the check fails and posts a separate notice explaining the label, unless that notice already exists.
• Adding or removing the label waits for the current CI run to finish, then reruns the original Size Check and its dependent jobs.

Also removing the old absolute size limits and the weekly size limit update workflow, since these are no longer needed now.

Fixes #21813

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[sentry]

Bug: The removal of explicit number coercion for result.size may cause bytes.format() to fail if the size field from the JSON output is a string.
_{Severity: MEDIUM}

Suggested Fix

Restore the explicit number coercion to ensure the size property is always a number before being used, especially by external library functions like bytes.format(). Change size: result.size back to size: +result.size.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: dev-packages/size-limit-gh-action/utils/SizeLimitFormatter.mjs#L80

Potential issue: The explicit number coercion for the `size` field, previously done
using `+result.size`, has been removed. While JavaScript's automatic type coercion
handles arithmetic operations like subtraction correctly even with string numbers, the
`size` value is also passed to the `bytes.format()` method from the `bytes-iec` library.
This library likely expects a numeric input and may return `null` or incorrect output if
`size-limit`'s JSON output provides the `size` value as a string. This removal of
defensive type coercion introduces a risk of silent failures or incorrect formatting in
the size report.

_{Did we get this right? 👍 / 👎 to inform future reviews.}