refactor(supervisor): converge boot and restart load pipelines

[mfw78]

What

Converges the boot and restart load pipelines for modules and providers into shared
helpers instead of duplicated skeletons: a shared store-build helper parameterised by
role, shared instantiate/install helpers reused by both the initial load path and the
lifecycle revive path, and a merged Seed type replacing the separate ModuleSeed and
ProviderSeed literals.

Confines run_init to the instantiate seam, drops the boot-only refuel that the dispatch
path already repeats, and adds a regression test pinning the boot-kills vs
restart-defers init verdict so the converged pipelines keep that distinction visible at
the call sites.

Why

Part of #94

The boot and restart load paths carried twin implementations that had drifted apart in
small, hard-to-audit ways (duplicated store-build logic, duplicated seed literals, a
refuel step only one path needed). Converging them onto shared helpers removes that
duplication while keeping the one deliberate behavioural difference, dead-permanent
vs restart-defers on init, explicit at both call sites instead of buried in copies.

Testing

• cargo nextest run -p nexum-runtime supervisor: 94 passed, 0 failed, including the_same_init_fault_kills_at_boot_and_only_defers_on_restart pinning the boot-kills vs restart-defers verdict.

AI Assistance

Implementation by Claude (Fable 5), red-team review by Claude (Opus 5), PR by Claude (Sonnet 5).