fix(sync): item-3 rewrite works on BSD awk and no longer reports a rewrite that failed

[MendixMau]

Fixes #426

What changed and why: _ea_rewrite_item3 passed the multi-line item-3 replacement through awk -v. Stock macOS awk rejects that ("newline in string", exit 2), so the file stayed unchanged while sync printed "Rewrote" and counted a change, on every run. The replacement is now streamed from a temp file (same pattern as the Q9 repair). The helper returns 2 when awk fails, and the caller prints a WARN: instead of "Rewrote". Other awk -v sites were checked: all pass single-line values, nothing else to fix.

Before / after (bash tests/wave2/test-exec-approval.sh, macOS awk 20200816, also with PATH=/usr/bin:/bin):

	PASS / FAIL
master	37 / 6
this branch, original 43 checks	43 / 0
this branch incl. 3 new assertions	46 / 0 (both PATHs)
master incl. the 3 new assertions	38 / 8 (new ones fail on old code)

New assertions use an awk shim that fails the replacement call: no "Rewrote", a WARN is printed, file unchanged.

Impact

Row	Answer
Process	None: no new human step or gate; a failed rewrite now prints a WARN.
Dependencies	bin/sync-project.sh, tests/wave2/test-exec-approval.sh, CHANGELOG.md. No new artifact reads. Uses mktemp and awk getline, already used by the Q9 repair.
Context cost	none: no routed or always-loaded file touched
Behaviour	n/a: scripts only
Runtime	n/a: one extra temp file per rewrite, not on a hot path

Field evidence: observed on macOS (awk 20200816) with synthetic fixtures only. No project data.

• No client data anywhere in the diff
• Size cap respected (3 files)
• Test tier: T1, test-exec-approval.sh on macOS BSD awk
• CHANGELOG line added

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