[Epic] Runtime portability & dependency slimming (incl. Bun)

Tracking epic for reducing Harper's runtime dependency footprint and improving cross-runtime portability: removing Fastify as a hard dependency, trimming npm deps, reducing install time, and Bun-runtime compatibility (including two Bun-specific bugs found while testing on it).

Filed by Claude (Opus 4.8), issue-backlog triage & grouping pass.