docs: cross-link general docs to the discipline demand docs

Tighten the router so race-specific reasoning in the general docs points to
the matching demand profile instead of speaking in a road/TT-default voice:

- periodization.md: anchor the build phase's 'race-specific top-end' to the
  matching demand doc (tt / road-racing / mtb-xco).
- durability.md: show how fatigue resistance manifests per discipline.
- data-confounds.md: extend the intermittent-HR caveat to road racing/crits;
  note TT as the steadier exception where HR is more usable.
- strength-for-cyclists.md: link the explosive/sprint transfer to road-racing
  and sustained-power transfer to TT.
- vo2max-intervals.md: match rep length to event (short=surges/road+MTB,
  long=sustained/TT); extend the HR caveat to road racing.

All internal links verified resolving.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TqrhBhC3GEcKyaw8RTk8G6

references/durability.md

@@ -72,5 +72,7 @@ Use `../assets/field-test-durability.md`. In brief:
 
 Durability matters for both road/TT and MTB, but **keep absolute numbers discipline-specific.** The
 1,500 kJ anchor is a sensible generic default, not a validated cross-discipline constant; a masters
-marathon-MTB rider and an elite road pro do not share a durability dose. See `mtb-xco-demands.md`
-for how fatigue resistance shows up specifically in off-road racing.
+marathon-MTB rider and an elite road pro do not share a durability dose. How fatigue resistance
+*shows up* differs by discipline: `mtb-xco-demands.md` (holding power through late off-road bursts),
+`road-racing-demands.md` (still having a sprint or matching-move after 4 h — a documented success
+determinant), and `tt-demands.md` (a long-TT threshold that doesn't decay to the line).