A composition-only layer installs, and its includes land beside it

[avrabe]

Two defects, both found building the covalent realm — the worked multi-realm
example: one pin over the PulseEngine toolchain and the bytecodealliance
component tools. It deposited and signed, and then neither command that had
to read it worked.

1. Install refused a layer made only of includes

layer 2026.09.0 carries no entry for platform x86_64-unknown-linux-gnu —
refusing to install a wrong-architecture toolchain

The fail-closed platform rule is right and stays. It asked the wrong
question: matched == 0 && !manifest.entries.is_empty(). A composition edge is
not a payload — it names another layer's manifest and the fetch loop
deliberately skips it — so a pure composition has zero entries that could
ever match a platform, and "none matched" is not evidence of a wrong
architecture. The check now counts payload entries.

a_layer_with_nothing_for_the_host_platform_fails_closed still passes. That is
the point: the rule is unchanged, only its question.

This is the deposit-side defect one stage down — deposit learned that a
composition-only layer is not an empty layer (#171); install had not.

2. The included realms nested inside the composing one — mine, from #177

fetch_included_layer used realm.effective_root(ctx.store.root()). That maps
a base root to <base>/realms/<fingerprint>, and ctx.store.root() is
already a partition, so a transitive install wrote:

realms/<covalent>/realms/<pulseengine>/…      <- nested
realms/<covalent>/realms/<pulseengine-wasm>/… <- nested

Install printed "fetched composed layer 2026.09.12 from realm 'pulseengine'"
and succeeded; the very next command said the layer was not installed, because
the composition walk looks at siblings.

ProjectCtx now carries base_root, named for what it is, so a caller cannot
reach for the wrong root without noticing which field they typed.

The result

composition: 3 layers —
  2026.09.0  (verified against realm 'covalent')  [pinned]
  2026.09.12 (verified against realm 'pulseengine')
  2026.09.0  (verified against realm 'pulseengine-wasm')

66 payload(s): 59 DISPATCHED, 7 HELD
  wac · wasm-tools · wit-bindgen-wrpc · wkg  →  pulseengine-wasm
  rivet · spar · synth · …                    →  pulseengine

Neither defect was reachable by unit test: one needs a layer with no payloads,
the other needs two realms in one store. Both are one command in the product.

Control: reverting the emptiness test to count every entry fails the new test.

Gate: fmt · clippy -D warnings · 1038 tests, 0 failed · trace-gate OK.

(trace-gate also caught me inventing REQ-COMPOSEONLY-001 from a PR title —
the marker now names REQ-COMPOSE-001, which exists.)

🤖 Generated with Claude Code

https://claude.ai/code/session_019TNtfRjLNhEz82G2ggeeNu