fix(wg_mgr): retry the initial direct-path handshake instead of one-shot

[fudio101]

Summary

Adapted from Csontikka/microlink@5bda1783 (issue #32), with the already-fixed active-clearing bug (PR #53, this same session) kept removed — the source commit's diff still cleared peer->active right after wireguardif_connect(), which PR #53 (issues #26/#28) proved permanently blocks that peer's direct-path handshake, since active is the master session-initiation gate, not a retry throttle. Porting this verbatim would have silently reintroduced a bug fixed two PRs ago.

• process_disco_pong() fired a one-shot direct-path handshake init gated by a single boolean latch (tried_initial_handshake): if that one init was lost or unanswered, the peer stayed permanently un-sessioned on the direct path even though further direct PONGs kept arriving. Replaced the latch with a retry timestamp (last_init_handshake_ms) so a dropped/unanswered init gets another try every 30s (INITIAL_HANDSHAKE_RETRY_MS) instead of giving up forever.

FORK_PRS.md row 13 (issue #32) marked done. Also documented that issue #36 (wake blocked sockets on stop) was investigated as part of this batch and isn't needed here: this fork's microlink_stop() (from issues #21/#22's PR #47) never frees context until ml_join_tasks() proves every worker exited, and ml_derp.c's blocking loops already call ml_shutdown_pending() on every SO_RCVTIMEO-bounded iteration to bail out cooperatively — the source commit's UAF doesn't exist in this fork's architecture, so no port planned (row left open for now).

Closes #32

Test plan

• idf.py build not runnable in this sandbox — needs a real ESP-IDF 6.x build to confirm.
• grep confirms tried_initial_handshake is fully gone, replaced by last_init_handshake_ms/INITIAL_HANDSHAKE_RETRY_MS.
• Confirmed dev->peers[...].active = false did not reappear anywhere in ml_wg_mgr.c (the other active = false hits found by grep are unrelated — peer-table eviction and DISCO pending_probes, different structs).
• FORK_PRS.md diff touches only rows 13 (Retry direct-handshake init every 30s instead of one-shot latch (mined from Csontikka/microlink) #32, done) and 17 (Shutdown derp.sockfd/coord_sock during microlink_stop() (mined from Csontikka/microlink) #36, investigation note).

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