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/data-confounds.md

@@ -16,8 +16,10 @@ factors. Use HR only where it is genuinely informative:
 - **Aerobic decoupling on steady rides** (Pw:HR drift) — valid only on steady efforts.
 - **Recovery trends** (RHR, HRV) — as *trends*, screened for the confounds below.
 
-Do **not** use HR to prescribe or judge intensity for **intermittent** efforts — see the MTB
-dissociation below.
+Do **not** use HR to prescribe or judge intensity for **intermittent** efforts — this covers MTB
+(`mtb-xco-demands.md`) *and* mass-start road racing/criteriums (`road-racing-demands.md`), both of
+which are surge-driven. See the dissociation below. Steady solo efforts (TT — `tt-demands.md`) are
+the exception where HR is more usable, though still secondary to power.
 
 ## The confounds (screen every HR-based signal against these)
 
@@ -38,7 +40,8 @@ In intermittent off-road riding, **HR, power, and VO2 dissociate**: VO2 stays el
 where power drops, and HR lags the short bursts that define the effort. In XCO, %VO2max did **not**
 correlate with %HRmax or %MAP (Hays et al. 2018, *Front Physiol* 9:1062; PMID 30158873;
 DOI 10.3389/fphys.2018.01062). **For MTB, HR is unreliable for intensity — power governs.** See
-`mtb-xco-demands.md`.
+`mtb-xco-demands.md`. The same intermittent logic applies to mass-start road racing and criteriums
+(`road-racing-demands.md`): HR lags the decisive surges, so prescribe and read those from power too.
 
 ## Do NOT hardcode naive rules