docs: plan nsec sign-in, and the ten call sites that make it small

An nsec is a BIP32 leaf of the seed at m/44'/1237'/0'/0/0, and the
derivation runs one way, so an nsec can never have a wallet behind it. What
makes the feature tractable anyway is that nothing in the app reads the
wallet except the nostr key -- ten sites, all the same expression -- so the
plan puts an Identity in front of the wallet and gives an nsec an identity
with no node behind it.

Eight phases: the identity type; a sibling key file in the library under the
one keystore alias Android will accept; listing and starting both kinds; the
sign-in screen for a phrase or an nsec, deduped on pubkey rather than wallet
id; indexer relays and a not-found exit for the sync that today asks one
relay and never finishes; recovery for a key with no phrase; tests; rollout.

Two findings along the way are recorded as pre-existing rather than new:
loadNostrProfile(startupRoute) keys off a Profile row a placeholder account
does not have and answers Landing while observeProfile answers the sync
screen, and the library's LocalKeyManager.nostrPublicKey() returns the
33-byte compressed key, not a nostr key.

Replayed onto Mantra by docs/curated-to-mantra.md: README.md: line-set three-way merge, both sides' additions kept and this commit's deletions applied.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Pulled-From: curated/curated@daae63d5c5

docs/README.md

@@ -16,6 +16,7 @@ silent, or a decision that looked arbitrary and was not.
 | [mls-skipped-keys.md](./mls-skipped-keys.md) | why a group event that arrives a moment late is dropped for good, which flows trigger it, the quartz fix, and the partial mitigation in this app |
 | [long-running-sync.md](./long-running-sync.md) | the chat subscriptions that stay open instead of pulling once per screen — why the request queue could not simply hold one, and how the group filter follows the room list |
 | [dead-code.md](./dead-code.md) | code in the sync and relay stack that nothing calls, why each piece is still there, and which of it is a bug rather than a leftover |
+| [nsec-sign-in.md](./nsec-sign-in.md) | signing in with an existing nostr key — why an nsec can never have a wallet behind it, the ten call sites that make it small, and the sign-in machine that was already built and unreachable |
 | [jvm-target.md](./jvm-target.md) | what desktop support cost, phased — why the native chain was already done, why an empty source set in our phoenix fork was the real blocker, and why DAO tests need none of it |
 | [material-design-conformance.md](./material-design-conformance.md) | what the M3 foundations actually require, measured against all 43 screens — the colour pairing that renders the app's own proposals invisible, and eight phases that put the decisions back in the theme |
 | [curated-to-mantra.md](./curated-to-mantra.md) | pulling the Curated fork's thirty-nine commits back under Mantra's names — which lines of work to take, the three decisions, and a measured way to replay a twice-rebranded history without touching seven hundred files by hand |
@@ -39,3 +40,7 @@ jvm-target note first if you want to know why its adaptive-layout phase exists.
 The curated-to-mantra note is a phased plan that has not been built, though its dry
 run has: it is about the repository rather than the app, and reads alone, except that
 its first decision leans on the derivation note's one rule.
+The nsec sign-in note is a phased plan that has not been built; it inherits the
+key-storage decision from the jvm-target note and drives the navigation state
+machine `NavigationViewModel.processLocalAccount` implements, so read it with the
+code open.