fix: include 17 missing icons in the SVG sprite and correct the validation logic

[rstaib]

Summary

Note: reordering mapping.json flips the canonical name for these 17 in the generated codiconsLibrary.ts (e.g. symbolVariable is now the definition, variable its alias). All existing names still resolve.

17 icons whose SVG file is named after a secondary alias were silently dropped from the generated SVG sprite.

The sprite generator located each icon's file via its primary alias (mapping.json aliases[0]). For these 17 icons, the file on disk is named after a different alias. For example, the file is symbol-variable.svg, but the codepoint's primary alias is variable, and no variable.svg exists. As a result, fs.existsSync failed and the entire codepoint (every one of its aliases) was excluded from the sprite.

The same primary-vs-filename mismatch meant metadata.json was keyed by names that don't match any SVG file, while the preview looks metadata up by filename.

check-metadata.js reported 100% coverage throughout, because it only validated the primary alias, so it never caught the gap.

Fixes #480

Affected icons (file name ← previous primary/metadata key)

SVG file	was keyed/ordered as
circle	circle-outline
desktop-download	clone
ellipsis	more
github	logo-github
go-to-file	symbol-reference
lock	gist-private
reply	mail-reply
repo-forked	gist-fork
sign-in	log-in
sign-out	log-out
star-empty	star
symbol-array	array
symbol-event	zap
symbol-namespace	symbol-module
symbol-variable	variable
sync	repo-sync
warning	alert

[rstaib]

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