docs(player): record who owns the volume, and what it gave back

CLAUDE.md's volume paragraph ended "#64 asks for it to be gone on
Android outright, which is a platform question the frontend cannot
currently ask", which is no longer true -- it can, and the paragraph
now says why the answer is a capability rather than a viewport and what
that costs.  The mediacontrols entry gains the corollary: on that
platform "the user's level" is a constant, and the duck is the one
thing that may still move the output.

NOTES.md carries the measurements: the per-element budget at 424x439
before and after, the :host([hidden]) specificity trap, the fact that
the bar's centring survives the control going away, and what no tier
here could check.

.planning/NOTES.md

@@ -4573,3 +4573,79 @@ fault in it, and it is filed as #172 with the per-element budget. #64
 umbrella; folding shuffle and repeat back onto the primary row was
 considered and rejected — it buys 52px, leaves the art at 91px, and
 costs a third arrangement of the same five buttons.
+
+## The volume is not ours on Android, and the predicate could not be a width (measured 2026-08-21)
+
+#64 asked for the in-app volume control to be absent on Android. Its
+first Finding said `volume-control` "already stands down at narrow
+widths", which was true of one of its two copies and is why the issue
+had been read as nearly done. The bar's copy goes by width; the
+full-screen view's copy was deliberately kept, with a comment saying a
+slider does belong there.
+
+**The crux was platform versus width, and three options were on the
+issue.** What settled it is that the *backend* half of the same issue —
+pin the level at 1.0 — makes a width rule wrong on the platform the
+issue is about: an Android tablet at >=600px gets the bottom bar, and
+the bar's slider would then move a level that is pinned. That is a
+control that cannot act, which `library-status-indicator` already
+settled is worse than none. The same rule is wrong the other way below
+600px, where a narrow desktop window has no hardware keys.
+
+So the frontend asks the player — `SystemOwnsVolume` — and the answer
+is right at every width in both mount points. **The predicate is named
+after the capability rather than the platform**, which is what makes it
+testable: only `platformOwnsVolume` is behind a build tag, in two files
+that declare nothing else, and everything else is decided against a
+field a Go test sets either way. `frontend/test/components/
+volume-ownership.test.ts` stubs the binding and so exercises the
+*Android* rendering on an ordinary Linux runner; both of its tests were
+confirmed to fail on the build before the change.
+
+**Measured at 424x439, by flipping `platformOwnsVolume` to true in the
+`!android` file and rebuilding** — the real binding, the real store, the
+real component, everything except the tag:
+
+| element | before | after |
+|---|---|---|
+| header | 48 | 48 |
+| **album art** | **39** | **68** |
+| title / artist / album | 63 | 63 |
+| transport (seek + controls + volume) | **172** | **143** |
+| — seek bar | 19 | 19 |
+| — player-controls | 116 | 116 |
+| — volume-control | 21 | **0** |
+
+29px, which is the 21px control plus the 8px flex gap it stops drawing:
+a gap is only painted between boxes, so `:host([hidden])` costs the
+transport nothing rather than leaving a hole. That is #172's "~30px of
+pure gain" confirmed, and the art is 74% larger. It is still the
+second-smallest thing on the screen, which is #51's evidence.
+
+Three smaller things worth keeping.
+
+**`:host([hidden])` has to be written down.** The UA's `[hidden]`
+rule is `display: none`, but `volume-control`'s own `:host` sets
+`display: inline-flex` and outranks it — so setting `hidden` alone
+hides nothing. Same family as the nested-`#queue-button` specificity
+trap from the session before.
+
+**Rendering `nothing` and hiding the host are two different
+assertions**, and the component test makes both: an empty shadow root
+is what stops a by-role or positional query finding a button that
+cannot act, and `hidden` is what stops the host occupying space. Either
+alone passes on a build that gets the other wrong.
+
+**The bar's centring survives the control going away.** #23's outer
+columns are the same `min()` expression rather than content-sized, so
+at 900px with the volume gone the bar's centre, `audio-player`'s centre
+and `player-controls`' centre are all 450 — checked, because "the
+transport is centred with a slider bolted to one side" is the fault
+that rule exists for and removing the slider is the obvious way to
+re-break it.
+
+**What no tier here can check**: the constant itself, and ducking
+against a real audio-focus change. The first is a source sweep
+(`TestPlatformVolumeOwnershipIsDeclaredOncePerPlatform`), the second is
+`TestSystemVolumeStillDucks` against the arithmetic. Neither is a
+device, and no device was attached.