docs: land the saga guardrails on the milestones they constrain

Applies the seven-campaign saga direction spec (021c248) to the docs it
was written to protect. No new milestone, no new content, no BOM line.

docs/roadmap.md — a "Guarded direction" section, and a ⛨ saga note on the
four milestones that would otherwise foreclose the shape:
  M3  don't hardcode the title flow so a saga entry path can't be added
  M4  keep construct()'s `creation` a plain Dictionary; the scene must not
      become its only producer
  M5  the level curve is no longer open — L20 across 7 tiers; enemy stat
      blocks carry tier; no scaling multiplier, ever
  M9  the save is two objects (Saga + Campaign); humiliations move to the
      Saga or they die with the campaign

races-and-classes spec — the deferred "XP curve → M5" line now inherits the
ceiling rather than choosing one.

canon-roadmap — what the saga asks of content: a tier field when the
item/quest namespaces land, and a fixed-vs-variable mark on each BOM line
when the BOM is written. Tiers 2-7 and the other six kings go to the
Backlog.

Content build --check green; no generated content touched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

content/lore/canon-roadmap.md

@@ -33,9 +33,32 @@ and its GUT tests in `client/tests/unit/test_content_db.gd`. Deleting generated
 content — a whole namespace of topics/npcs/canon — is a **client change**, not
 just a content change; check `client/` before purging.
 
+## The seven-campaign saga — what it asks of content
+
+The saga direction spec
+([`docs/superpowers/specs/2026-07-12-character-reuse-seven-arc-design.md`](../../docs/superpowers/specs/2026-07-12-character-reuse-seven-arc-design.md))
+authors **no content** and adds **no BOM line**. It asks two things of the content
+track, both of which land inside work already sequenced:
+
+- **`tier: 1..7` on the entry envelope**, validated by the build tool — added at the
+  moment the `item` and `quest` namespaces land (already an M7/M8 prerequisite in the
+  content-track design). Schema, not content.
+- **When the Bill of Materials is written, mark each line fixed vs variable.**
+  *Fixed substrate* (identical in every campaign): currency, the price table,
+  abilities, callings, the cosmology, the Seven, the disposition ladder.
+  *Per-campaign variable*: the town, its NPCs, their gated knowledge, the quest
+  chain, the king. This tells you which BOM lines town #2 reuses verbatim — useful
+  now, and the exact split v2 generation will need.
+
+Pleasant accident: **the Greywater slice is already campaign 1** — tier 1, patron
+Ghaul the Yoke. Nothing in the content-track design changes.
+
 ## Pending (not yet authored)
 
 - Per-town knowledge that grounds the cosmology: possessed NPCs, blood-harvest
   sites, shrine fronts — authored as gated `rumor`/`fact`/`secret`.
 - The Barbarian ⚔ calling name (`[CANON-TBD]`); the Bloodsworn's patron-choice
   detail (M4). Regions/towns of the Margreave beyond the scaffolding.
+- **Backlog (saga, tiers 2–7):** the other six kings as campaign patrons; tiered
+  enemy families; the three story-shapes (cult cell → cult that owns a city → the
+  king). Parked, not lost.