refactor: group internal non-product targets under Sources/_support/

[zzal]

What

Moves the three non-product build-scaffolding targets — SwiflowCSSCore, SwiflowEmbedders, MacroConsumerChecks — under Sources/_support/.

Why

Sources/ had 16 flat directories, implying 16 modules to learn. Only 12 are products (importable libraries/executables) plus SwiflowMacrosPlugin (the compiler plugin, kept visible by convention). The other three are internal scaffolding: a zero-dep CSS-rules leaf, the codegen emit logic, and a plain-swift build compile gate. Grouping them under _support/ (sorts to the bottom) makes the top level read as the public catalogue.

Scope

Pure relocation via git mv (history-preserving renames):

• Only the three path: strings in Package.swift change.
• Target/product graph is byte-identical: 29 targets, 12 products.
• No CI workflow, script, or test target references these paths (verified — CI's Sources/ refs touch only product targets + SwiflowCLI).
• Dropped the now-stale // Sources/<Target>/File.swift path-echo header lines from the moved files.

Verification

• swift build clean.
• Full suite: 1922 tests / 356 suites passed (8 pre-existing withKnownIssue markers, no regressions).

🤖 Generated with Claude Code

[github-actions]

📦 Bundle size

Artifact	Baseline	This PR	Δ
App.wasm	5.06 MB	5.23 MB	+3.46%
App.wasm (gzip)	1.80 MB	1.87 MB	+3.56%
JS runtime	54.5 KB	54.5 KB	+0.00%
JS runtime (gzip)	11.3 KB	11.3 KB	+0.00%
Total (gzip)	1.81 MB	1.88 MB	+3.54%

✅ Within budget (≤5% growth allowed).

_{Baseline: Swift 6.3, WASM SDK 6.3-RELEASE, measured 2026-06-18.}