[V2:16] Add Targeted Serve Rendering

[ritorhymes]

Part of V2: Multi-repo local build system.

Summary

This PR completes the targeted rendering path by extending V2:15's build-only proposal selection to local dirty serve. serve --only <NUMBER>... and workspace [render].only now resolve through the same OnlyRenderPlan and CLI-over-config rules as build while remaining unavailable for check, --clean, and --remote-siblings; the prepared pipeline passes that plan into dirty serve sync so omitted proposal markdown/assets are filtered out, selected deletions are preserved through previous/current dirty-path unions, and affected selected proposal markdown, selected asset markdown, and retained non-proposal markdown are incrementally preprocessed with missing-path tolerance. Theme sync remains independent of targeted proposal selection, so review should focus on serve applicability and rejection rules, dirty-path filtering, deletion propagation, incremental preprocessing across the three retained categories, and preventing omitted content from being reintroduced while Zola is running.

• Extend targeted selection from build to local dirty serve.
• Accept build-eips serve --only <NUMBER>... and reuse the existing strict proposal-number parser.
• Apply workspace [render].only defaults to local dirty serve as well as local dirty build.
• Keep targeted selection unavailable for check, --clean, and --remote-siblings.
• Pass OnlyRenderPlan from the prepared runtime pipeline into dirty serve synchronization.
• Filter active-repo dirty paths so omitted proposal markdown and assets are not reintroduced during targeted serve.
• Preserve selected proposal deletions by syncing affected paths from the previous and current dirty sets.
• Add incremental targeted markdown preprocessing for selected proposal markdown, selected asset markdown, and retained non-proposal markdown.
• Add missing-path tolerance for dirty serve preprocessing so deleted selected paths do not abort the watcher.
• Update the generated workspace guide with targeted build/serve usage.

Review Notes

This PR completes the targeted rendering path started in V2:15. V2:15 made targeted build work after preparing the full materialized content tree. This PR adds the serve-specific piece: keeping the already-running dirty serve watcher aligned with the same selected proposal set.

--only is accepted on default local dirty build and serve. check does not accept it. Execution policy rejects --only with --clean and --remote-siblings.

Workspace [render].only is now a default for applicable local dirty build and serve runs. CLI --only still overrides config values, and missing or empty config values still mean full rendering.

The prepared pipeline already builds OnlyRenderPlan before Zola starts. This PR passes that plan into serve_sync_config, so dirty active-repo sync can use the same selected/unselected policy while Zola serve is running.

Dirty serve sync filters active-repo paths before computing the affected path union. That keeps dirty changes under omitted proposal markdown or omitted proposal asset directories from being copied into the materialized repo. Selected proposal assets still sync normally.

The affected path set is the union of the previous filtered dirty paths and the current filtered dirty paths. That preserves deletion handling for selected proposal files that were dirty in the previous scan but are now gone.

markdown::preprocess_paths is added for incremental serve updates. It preprocesses only affected selected proposal markdown, selected proposal asset markdown, and retained non-proposal markdown such as _index.md, while still using OnlyRenderPlan to rewrite links and requires entries to omitted proposals.

Dirty serve preprocessing tolerates missing paths because a filesystem event may represent a deletion. Full build preprocessing still treats missing paths as errors.

The markdown timestamp fallback handles dirty files that do not yet have a Git commit timestamp. When git log returns no timestamp for the current path, preprocessing falls back to the filesystem modified time.

This PR does not change theme sync behavior. Local theme sync remains independent from targeted proposal selection.

Verification

• Review src/cli.rs for serve --only parsing, invalid selector rejection, and continued rejection on non-targeted commands.
• Review src/execution.rs for build/serve targeted selection applicability, CLI-over-config precedence, and rejection of unsupported local/remote/clean modes.
• Review src/pipeline.rs for passing OnlyRenderPlan into serve_sync_config.
• Review src/serve.rs for dirty-path filtering, affected-path union behavior, selected deletion handling, selected asset sync, and prevention of omitted proposal reintroduction.
• Review src/markdown.rs for preprocess_paths, missing-path tolerance, incremental targeted preprocessing, retained non-proposal markdown handling, lexical link normalization before public URL lookup, and filesystem timestamp fallback.
• Review src/workspace_doc.md for targeted build/serve user guidance.
• Review serve tests for targeted sync config, filtered dirty paths, selected deletions, omitted markdown/assets, selected asset sync, selected markdown preprocessing, retained non-proposal preprocessing, and deletion propagation.