fix(node): the leaseless bound refused the music player

Reported the day MNP 3.0 shipped: playing a track answered "Too many files open
at once without a transfer. Download this one instead of previewing it."

§3.4.1's bound of two was reasoned about *viewers* — a photo viewer shows one
photo, a preview modal one document, and the second is for prefetching the next.
It forgot the music player, which warms a read-ahead window: `prefetchDepth()`
returns 5 on Wi-Fi and 3 otherwise, so playing an album has six files in flight
and the fourth was refused. Browsing a group is never subject to a transfer slot
— that is a stated requirement, not a tuning parameter — and a constant nobody
had checked against the client broke it.

Twelve now: six for the music read-ahead at its widest, two for a photo viewer
and its own prefetch in the same session, the rest as headroom. Generosity is
cheap here and refusal is not — this is a fairness control among cooperating
clients, not a security boundary, so a client that lies gets twelve files at a
time instead of its member cap, bounded and audited, while refusing a legitimate
read breaks the requirement outright.

And the number is now derived rather than chosen: a test reads `prefetchDepth()`
out of the shipped player and fails if the node's bound no longer covers it, so
widening the client's read-ahead breaks the build instead of reaching a person.
Checked by widening it: "the music player reads 21 files ahead and the node
admits only 12".

Three cases that hard-coded "two then refuse" now set their own limit — they are
about the mechanism, and the shipped number moves with the client.

Node suite 1210 passed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGdheDLxGReuKHga3BtST

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