docs(apps): rewrite shared-base-runtime proposal for the converged seam

The original proposal modeled an 11-capability RuntimePlatform/setPlatform
singleton over a "19 require-coupled files" core. After Deno's move to node:
compat and this branch's convergence, the genuine seam is 5 files and the base
is the whole tree minus a thin adapter. Re-derive from the current tree:

- 5-file seam (require/loader-hook, stdoutTransport, construct, error-handlers,
  main); 52 of 57 files are base.
- Injection model generalizes the existing Transport/setTransport pattern into
  independent setters (transport, sandbox require, sandbox globals) wired in each
  adapter's main.ts; prepareEnvironment hoisted to bootstrap.
- Module resolution framed honestly as a non-injectable adapter responsibility
  (host-resolution bootstrap) split from the injectable sandbox require.
- Package shape P1: base-runtime own dir + own runtime tsconfig, consumed via the
  existing @rocket.chat/apps prefix (no new resolver wiring); P2 documented as
  future work gated on a nodenext migration.
- Test strategy: base suite under node:test, thin per-adapter seam suites, no
  --no-check base re-run on Deno.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: d-gubert