Invisible activation: single generated numan_init.nu bootstrap file

[tonythethompson]

Problem

Activation already writes Numan-owned autoload state safely (nu/autoload.rs, state/autoload_state.rs, journal recovery, drift detection), but the user-facing story still implies config.nu awareness of individual activate/deactivate/rollback operations. For Numan to feel invisible, the user should touch config.nu exactly once, ever.

Goal

numan owns a single generated file (e.g. <root>/nu_state/numan_init.nu) that the user sources from config.nu with one line, added once. Every numan activate, numan deactivate, and numan snapshot rollback atomically regenerates that one file to reflect current state — no other config.nu edits, ever.

Scope

• Define numan_init.nu as the single top-level file config.nu sources; it aggregates/sources the existing per-kind generated autoload files rather than replacing their separation.
• Regeneration must use the same atomic-write guarantees as the rest of util/fs_safety.rs and existing autoload state writers — no partial/torn file ever visible to Nu.
• A one-time setup step (extend numan setup or numan doctor --fix) that idempotently inserts the single source line into the user's config.nu, without disturbing surrounding content.
• Drift detection: if numan_init.nu doesn't match what current lockfile/snapshot state would generate, numan doctor should flag and offer --fix, consistent with existing drift-detection patterns from Phase 3/4.

Acceptance criteria

• After the one-time config.nu edit, a full cycle of install → activate → snapshot → rollback → deactivate requires zero further manual config.nu changes.
• numan doctor detects a hand-edited or stale numan_init.nu and can restore it via --fix.
• Real-Nu acceptance test verifies a fresh Nu session sourcing numan_init.nu sees the exact activation state Numan's lockfile/snapshot claims.

Non-goals

• Managing any part of the user's config.nu beyond the single source line.
• Replacing the existing per-kind autoload file architecture — numan_init.nu is a thin aggregator on top of it.