Document Release 1 adapter compatibility

docs/USER_MANUAL.md

@@ -9,6 +9,12 @@ DocForge 1.0.0 is the first stable product release. It includes the project-scop
 CLI and MCP query surfaces, hash-approved proposal application, generic and project-owned adapters,
 declared rendering, and the Nodes/Flow/Web visualization model documented below.
 
+Later incremental-compiler capabilities are additive. A Release 1 adapter with only
+`load_projection()` remains valid and follows the same complete-rebuild path. No existing project
+descriptor, canonical document, changeset, or adapter must be rewritten. Source-scoped caching and
+lazy logic projections activate only for adapters that explicitly implement the optional
+incremental methods while retaining the full loader as a fallback.
+
 ## Features
 
 - Project-bound Markdown and TOML documentation graphs with stable node IDs.
@@ -505,6 +511,10 @@ implement the optional source-scoped manifest and extraction contract. DocForge
 sources, reuses unchanged facts, reparses changed sources and their reverse dependents, validates a
 complete candidate graph, and publishes the index atomically.
 
+DocForge detects this capability structurally. An adapter without both `load_manifest()` and
+`extract_source()` remains on the Release 1 path. Its behavior and query results are unchanged, but
+it does not receive incremental performance until it opts in.
+
 Build results report cache hits, reparsed sources, invalidated sources, deleted sources, and total
 sources. A full projection remains the fallback and equivalence oracle.