suunto_nautic: skip foreign frames on a contended link instead of erroring

[latishab]

Problem

The Nautic/Ocean BLE link is single-central, but another client can still hold a session on it. When a user leaves the official Suunto app running with a live logbook subscription, that subscription's Whiteboard 0x07 subscribe-result frames leak onto the shared link (on a different handle from our fetch).

Both fetch loops (suunto_nautic_device_paginated_fetch and suunto_nautic_short_fetch_frame) skipped non-DATA frames but capped skips at 8, then returned DC_STATUS_DATAFORMAT. A burst of that foreign traffic blows through the cap immediately, so the download aborts with a misleading data-format error and the user gets zero dives.

Confirmed mechanism (decompiled libmds.so): 0x07 routes through Whiteboard::handleClientResult with (type & 0xe) == 6, i.e. it is the Suunto app's subscription traffic, not a response to our request. Field-confirmed with multiple paying users on production; every case clears once the Suunto app is fully closed.

Change

• Distinguish foreign-but-well-formed frames (0xA5 magic, but not our 0x05/handle) from genuinely malformed/truncated ones.
• Skip foreign frames freely (bounded only by MAX_FOREIGN_SKIPS to guard against an unbounded loop); keep the tight MAX_MALFORMED_SKIPS = 8 cap for malformed frames → DATAFORMAT.
• When our DATA never gets a turn on a saturated link, the dc_iostream_read times out first, so the failure now surfaces as a clean DC_STATUS_TIMEOUT instead of DATAFORMAT.
• Factored the two frame checks into small suunto_nautic_frame_wellformed() / suunto_nautic_frame_is_our_data() helpers used by both loops.

What this does and does not do

• It does not make a contended download succeed — the user still must close the app holding the subscription (that remains the real fix, and we ship an in-app hint for it).
• It does turn a corrupt-data error into an honest "couldn't read the device" outcome, never truncates a good download on a brief foreign burst, and helps any consumer of the driver, not just apps with an app-side heuristic.

Downstream note

The terminal status for the pure-contention case changes DATAFORMAT → TIMEOUT. Any consumer keying a "close the Suunto app" hint off DATAFORMAT + no dives should broaden it to include TIMEOUT (tracked on our side as a Currents follow-up to DEE-45).

Testing

• swift build clean; existing swift test suite green (25 tests, 1 skipped).
• The fetch loops require a live device, so they're exercised via field testing rather than unit tests; the parser/tank-pressure suites are unaffected.