[V2:15] Add Targeted Build Rendering

[ritorhymes]

Part of V2: Multi-repo local build system.

Summary

This PR adds the first user-facing targeted rendering slice for V2: build-only proposal selection through build --only <NUMBER>... and workspace [render].only defaults. Selection resolves into ResolvedExecution::only with CLI values overriding config and applies only to local dirty builds (CLI --only is rejected on other forms; [render].only is silently ignored outside that scope); it builds an OnlyRenderPlan after source and sibling materialization, keeps references between selected proposals internal, rewrites links and requires entries for omitted proposals to public EIP/ERC URLs, and prunes unselected proposal content before Zola runs. It does not add targeted serve behavior, so review should focus on selection applicability, CLI-versus-config precedence, selected-versus-omitted reference handling, and pruning.

• Add build-only --only <NUMBER>... CLI selection for targeted local rendering.
• Add workspace [render].only defaults to .build-eips.toml and the starter config.
• Resolve targeted render selection into ResolvedExecution::only, with CLI values overriding workspace config values.
• Deduplicate selected proposal numbers while preserving the strict positive-number parsing from V2:13.
• Restrict targeted rendering to local dirty build mode; command-line --only is rejected outside applicable build forms, and config [render].only is ignored outside applicable local dirty builds.
• Build OnlyRenderPlan during prepared runtime setup and pass it into markdown preprocessing.
• Rewrite links and requires entries that point at omitted proposals to public EIP/ERC URLs.
• Preserve internal links and requires references between selected proposals.
• Prune unselected proposal markdown/directories before Zola runs.

Review Notes

This PR is the first user-facing targeted rendering slice. It consumes the proposal-selection foundation from V2:13 and applies it to build only. Targeted serve behavior is intentionally deferred to V2:16, where dirty serve sync and incremental preprocessing need additional filtering rules.

--only is accepted only on build. Execution policy applies it only to default local dirty builds and rejects it with --clean or --remote-siblings; serve and check do not accept it. Workspace [render].only is ignored unless execution resolves to a local dirty build.

Workspace [render].only is a default for applicable local dirty builds. Command-line --only overrides the config value. Missing [render], missing only, and only = [] all mean full rendering.

OnlyRenderPlan is built after active and sibling sources are materialized and merged into the prepared repo. That lets targeted selection operate on the actual content tree Zola will render, including sibling proposal content.

Markdown preprocessing uses the plan to keep selected proposal references internal and rewrite references to omitted proposals as public URLs. This applies to normal markdown links and proposal requires front-matter entries. Query strings and fragments on rewritten markdown links are preserved.

After preprocessing, unselected proposal files and proposal directories are pruned from content/. Non-proposal content such as _index.md remains available so the selected output still has the surrounding site structure.

Verification

• Review src/cli.rs for OnlyCliArgs, build-only --only parsing, invalid selector rejection, and rejection on serve/check forms.
• Review src/config.rs for [render].only, starter config serialization, config parsing, empty/missing render behavior, and invalid render-only values.
• Review src/execution.rs for targeted selection resolution, CLI-over-config precedence, deduplication, local dirty build applicability, and rejection of unsupported --only modes.
• Review src/pipeline.rs for OnlyRenderPlan construction after source materialization/merge, markdown preprocessing with the plan, and pruning before Zola runs.
• Review src/markdown.rs for targeted markdown filtering, external URL rewriting, requires rewriting, pruning-safe preprocessing, query/fragment preservation, and tests.
• Review src/editorial.rs only for the ResolvedExecution::only test fixture update; editorial behavior is otherwise unchanged here.
• Review src/serve.rs only for the non-targeted markdown::preprocess(..., None) call; targeted serve is deferred to V2:16.