docs(wam-clojure): fix lowered tier doc links

[s243a]

Summary

Fix the merged Clojure lowered-tier proposal docs so they render
correctly on GitHub and read more cleanly as a follow-up to PR #1696.

This PR:

• replaces machine-local Termux paths with repo-relative links
• clarifies how the lowered-tier proposal relates to the broader
  tiering doc
• tightens the routing heuristic for choosing between:
  • classic native lowering
  • lowered WAM
  • foreign/kernel lowering
  • full WAM fallback
• cleans up the phase / milestone wording in the proposal

Why

The previous merged proposal was architecturally sound, but its links
were written as absolute local filesystem paths. That breaks rendering
on GitHub and makes the proposal harder to review and reuse outside the
authoring environment.

This PR keeps the design intact while fixing the documentation defects
and improving a few places where the routing story was still too loose.

Files

• docs/proposals/WAM_CLOJURE_LOWERED_TIER_PLAN.md
• docs/design/WAM_PERF_OPTIMIZATION_LOG.md

Notes

This is still a docs-only follow-up. It does not start the lowered
emitter implementation yet. The next implementation-facing step remains:

1. scaffold wam_clojure_lowered_emitter.pl
2. add focused unit tests
3. wire the lowered tier into wam_clojure_target.pl