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/periodization.md

@@ -36,6 +36,12 @@ quantified finding.
 
 Work **backward from the A-race** (backward mapping). Each phase sets up the next.
 
+**Anchor the plan to the A-race's demand profile** — the "race-specific top-end" of the build phase
+means different things per discipline. Load the matching demand doc before shaping the build:
+`references/tt-demands.md` (sustained solo power, pacing), `references/road-racing-demands.md`
+(repeatable supra-threshold surges + a fatigued sprint), or `references/mtb-xco-demands.md`
+(intermittent, supra-MAP bursts). Don't build a TT engine for a criterium, or vice versa.
+
 | Phase | Primary job | Emphasis |
 |---|---|---|
 | **Base** | Raise sustainable CTL, aerobic base, **durability**, and strength capacity | High volume, mostly low intensity, small hard dose kept alive; build heavy strength |