Add §10 Resource fragmentation + send-resource flow

Closes Tier 1 #2. Without this, a client can't send any LXMF body
larger than LINK_PACKET_MAX_CONTENT ≈ 360 B, can't receive a NomadNet
page that doesn't fit in one MTU, and can't transfer files via rncp.

SPEC.md §10 (new): full Resource fragmentation protocol with citations
to RNS/Resource.py. 13 sub-sections covering preparation pipeline
(metadata prefix → optional bz2 → random_hash prefix → SHA-256 over
data||random_hash → link.encrypt of the WHOLE blob → part-split into
SDU-sized chunks → 4-byte map_hash hashmap with collision guard within
COLLISION_GUARD_SIZE = 2*WINDOW_MAX + HASHMAP_MAX_LEN), wire context
inventory (RESOURCE_ADV / RESOURCE / RESOURCE_REQ / RESOURCE_HMU /
RESOURCE_PRF / RESOURCE_ICL / RESOURCE_RCL), the msgpack dict for the
advertisement (t/d/n/h/r/o/i/l/q/f/m), the request payload format with
the hashmap_exhausted sentinel, the lazy-hashmap RESOURCE_HMU
continuation that lets large hashmaps avoid breaking small-MTU links,
the proof body
   resource_hash(32) || full_proof = SHA256(data||hash) (32)
returned in a PROOF-type packet, the sliding window dynamics
(WINDOW=4 → WINDOW_MAX_FAST=75 / WINDOW_MAX_VERY_SLOW=4 with rate
detection), multi-segment cutover at MAX_EFFICIENT_SIZE = 1 MiB - 1
with the lazy `__prepare_next_segment` pattern, and the
encryption-before-split layering that means a missing part can't be
decrypted in isolation.

flows/send-resource.md: 10-step chronology from RNS.Resource()
construction through advertise → req/parts loop → HMU continuation →
final RESOURCE_PRF → multi-segment fan-out, with a wire-byte ladder
diagram and a per-step source map.

Side fixes found while drafting:
  - SPEC.md §2.5 contexts table was wildly incomplete and had a real
    bug: KEEPALIVE was listed as 0xFD; upstream is 0xFA per
    RNS/Packet.py:87. 0xFD is actually LINKPROOF (the regular
    DATA-receipt context, §6.5). Replaced with the full upstream
    context inventory: NONE, RESOURCE_*, CACHE_REQUEST, REQUEST,
    RESPONSE, PATH_RESPONSE, COMMAND, COMMAND_STATUS, CHANNEL,
    KEEPALIVE, LINKIDENTIFY, LINKCLOSE, LINKPROOF, LRRTT, LRPROOF.
  - SPEC.md §6.5 reworded: "send back a PROOF packet (no context
    byte specifics)" → "send back a PROOF-type packet with
    context = LINKPROOF (0xFD)" for clarity.
  - The previously-numbered §10 "Test vectors" and §11 "Source map"
    are renumbered to §11 / §12 so the new Resource section lands in
    its correct protocol-stack position. agent.md §5 audit table
    updated accordingly.

flows/README.md status table updated; receive-resource.md added as
the next pending flow.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

agent.md

@@ -100,8 +100,9 @@ Initial confidence assessment (subjective, not authoritative — re-do this audi
 | §7.6 `TCPServerInterface.OUT` override | Source-cited; matches behavior observed in the mobile-app's local-transport experiments. |
 | §8 KISS / HDLC framing | High — both work in production on the reference clients |
 | §9.1–§9.8 Implementation gotchas | Each was a real bug that bit a real implementation. High confidence each is real; some lack formal test scripts. |
-| §10 Test vectors | The vectors themselves are verified; the test-vectors/ directory needs to be populated in this repo (currently empty placeholder). |
-| §11 Source map | High |
+| §10 Resource fragmentation | Source-cited from `RNS/Resource.py` against RNS 1.2.0; not yet runtime-verified in this repo's `tools/`. |
+| §11 Test vectors | The vectors themselves are verified; the test-vectors/ directory needs to be populated in this repo (currently partially populated). |
+| §12 Source map | High |
 
 **Concrete next-task list** for the agent picking this up: