Incremental builds beyond serve-mode

From LATER.md: incremental builds beyond serve-mode. Parked because full builds are fast enough at blog scale (the 76-page cherrybomb.dev site builds in seconds). Becomes real if someone brings a thousand-page site. Any design must preserve byte-determinism — an incremental build must produce the identical tree a full build would.

Start with LATER.md and inspect how incremental work is handled in serve-mode versus a full build. Define an approach for larger sites, then verify that an incremental build produces the identical tree and byte output as a full build.

Written by the indexing model from the issue text.