fix(page-tree): map NULL to U+FFFD in escapeCssIdentifier

[shuvamk]

Summary

escapeCssIdentifier is a vendored implementation of the CSS.escape algorithm (CSSOM §serialize-an-identifier), used to build class-kind selectors (styleRule.ts) and class-attribute tokens. It implements every step of the spec except step 1: "If the character is NULL (U+0000), then append U+FFFD REPLACEMENT CHARACTER to result."

The control-character branch deliberately starts at U+0001:

if (
  (codeUnit >= 0x0001 && codeUnit <= 0x001f) ||   // NULL (0x0000) excluded
  codeUnit === 0x007f ||
  ...

so a U+0000 matches no branch and falls through to the generic escaped += '\\' + char, emitting a backslash followed by a literal NULL byte. The browser's native CSS.escape maps U+0000 to U+FFFD instead.

Reproduction

Input (as code points)	Before	Native CSS.escape / spec
U+0000	\ + literal U+0000	U+FFFD
a, U+0000, b	a\ + U+0000 + b	a + U+FFFD + b

Every other step (control chars, DEL, leading digit, -+digit, lone -, allowed set, escape-as-char) already matches the spec — this is a single missing branch.

Fix

Add the U+0000 → U+FFFD branch as the first step of the per-code-unit loop, exactly where the spec places it.

The function had no direct test coverage, so this adds one alongside the fix — NULL, control characters, leading digit, digit-after-dash, lone dash, and passthrough — all matching native CSS.escape. The test is co-located under src/core/page-tree/__tests__/ and imports the helper relatively, so it neither expands the module barrel nor trips the deep-import gate.

Verification

• bun run build
• bun test (6337 pass, 0 fail; the NULL case fails on main and passes with the fix)
• bun run lint

Checklist

• Tests cover behavior changes.
• Docs were updated when behavior, config, deployment, or public surfaces changed — n/a (the docstring already claims CSS.escape parity; this makes the code match it).
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.