docs: correct the metric rule for "16 plus"

The spec said the metric follows the template the page renders. That is no
longer true for GIAS phase 6: a sixth-form college renders the primary
template but is matched, correctly, against secondaries. The rule is phase
group membership, decided once in is_secondary_phase — and the section's lede
noun comes from the school's phase rather than its template for the same
reason.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/superpowers/specs/2026-09-21-similar-schools-nearby-design.md

@@ -163,12 +163,25 @@ reader to assume otherwise.
 | `metric_key` | `rwm_expected_pct` or `attainment_8_score` — see below |
 | `metric_year` | the year the figure is from |
 
-The metric follows the template the page is rendering, not the neighbour's own
-phase, so a row of cards never mixes two scales. Primary and **all-through**
-pages use `rwm_expected_pct`, matching `PrimarySchoolSections`, which is the
-template all-through schools render with; secondary pages use
-`attainment_8_score`. Where the neighbour has no value for that key, the card
-reads "Not published" rather than falling back to the other key.
+The metric follows the phase side the school was *matched* on, not the
+neighbour's own phase, so a row of cards never mixes two scales. The secondary
+side uses `attainment_8_score`; the primary side uses `rwm_expected_pct`. Where
+the neighbour has no value for that key, the card reads "Not published" rather
+than falling back to the other key.
+
+Which side a school takes is decided once, in
+`similar_schools.is_secondary_phase`, by membership of `PHASE_GROUPS["secondary"]`
+minus all-through — never by testing for the substring "secondary", which misses
+`16 plus` (GIAS phase 6) and hands a sixth-form college the primary bucket.
+All-through is the exception in the other direction: `PHASE_GROUPS` lists it on
+both sides, but it takes the primary metric.
+
+This is usually the same thing as "the template the page renders", but not
+always. `computeSchoolFlags` decides the template with that same substring test,
+so a `16 plus` school renders `PrimarySchoolSections` while being matched — 
+correctly — against secondaries. The section therefore takes its lede noun from
+the school's own phase rather than from its template, or it would print "Other
+primary schools near <sixth form college>" above a row of secondaries.
 
 `tier` is carried explicitly rather than inferred from the contents of
 `shared`, because the frontend needs it for two separate decisions — whether the
@@ -355,6 +368,8 @@ synthetic frame rather than live marts:
 - tiers stop relaxing once three are found: four tier-1 matches never open tier 2
 - more than six qualifying schools returns the six nearest
 - an all-through school is offered on both phase sides
+- a `16 plus` school is matched against secondaries and colleges, never primaries
+- `is_secondary_phase` and `PHASE_GROUPS` agree on every GIAS phase value
 - fewer than two qualifying schools returns `[]`
 - distances match a hand-computed haversine for a known pair