Add per-instance hide/show for list items (t key)

[HoneyHazard]

⚠️ Full Disclosure: Drafted with AI assistance (Claude); reviewed by me briefly. I am neither a RUST developer nor pipewire expert. If I should stop making these PRs into your wonderful project, please let me know. ⚠️

That being said, I hope these can be helpful and useful additions that users could appreciate.

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Sometimes a node in the list is just noise you'd rather not see or monitor for the rest of the session - a stream you know you're not going to touch, a device you don't care about right now. There was no way to get it out of the way without closing the app or reconfiguring [[filters]].

This adds a t keybinding that toggles hiding the selected item for the current instance only (t for "toggle hide" - the same mnemonic carries over to Ctrl+t in the follow-up permanent-hide PR, #72):

• Hidden items sink to the bottom of their list, in a stable sort that otherwise preserves the existing order - both the still-visible group and the hidden group keep their relative order, so hiding something doesn't reshuffle everything else.
• Hiding the selected item releases the selection rather than leaving it pinned to a row that's about to sink to the bottom - it moves to whatever's next in line, falling back to whatever's previous if it was the last item, or to no selection at all if it was the only item. Un-hiding keeps the selection where it is - you're still looking at that item, there's nothing to release.
• Purely local to the running instance: nothing is written to disk, nothing is synced anywhere. Quitting and relaunching forgets it. (A durable, cross-instance-synced "permanent hide" tier is planned as a follow-up on Ctrl+t - this PR is deliberately just the local/session-only piece on its own.)

Default theme, row_hidden at its new default (a faint DarkGray tint).

Two new config keys make the look of a hidden row customizable:

• A new theme key, row_hidden: Style, patched onto a hidden row's text spans (title, target, volume%, and the Configuration tab's device/profile) - the same Style::patch() no-op-by-default mechanism the rest of the theme system already uses. Defaults to a faint DarkGray tint in the default theme and the DIM modifier in nocolor, so a hidden row is visually distinguishable out of the box, not just by its title prefix; plain leaves it unset on purpose, matching that theme's own "no styling anywhere" philosophy.
• A new char_set key, hidden_instance: String (default "[hide] "), prepended to a hidden item's title - so a hidden item stays identifiable even if row_hidden is left unset. The built-in char_sets and wiremix.toml's docs suggest an emoji alternative ("🙈 ") for anyone who'd rather not use text.

[themes.default]
row_hidden = { fg = "DarkGray" }  # this is now the default - shown for clarity

[char_sets.default]
hidden_instance = "🙈 "

Peak monitoring for hidden items is a separate, configurable concern from visibility. Hiding an item always removes it from the visible list, but whether it also stops being monitored is controlled independently by a new capture_hidden: bool option (default true), also available as --capture-hidden/--no-capture-hidden on the command line:

• It's a filter layered on top of the existing capture-eligibility rules (lazy_capture, filters, etc.), never a replacement for them.
• When true (the default), hidden items are captured under exactly the same rules as regular ones - hiding something never changes whether it's monitored unless you opt in.
• When false, hidden items are excluded from capture in addition to whatever else already excludes non-hidden items.
• When monitoring actually is suspended for a hidden item (hidden + capture_hidden = false), the peak meter is left completely blank instead of drawing the usual inactive-looking placeholder over a stream that will never receive samples - that placeholder otherwise reads as broken rather than intentionally off.

Both instances show the same [hide] Lavf61.7.102: record item, hidden with t. Left: default capture_hidden = true - the meter keeps reporting real levels. Right: --no-capture-hidden - the meter is blank because the capture stream itself is stopped.

# If false, hidden items are excluded from peak monitoring - an additional
# filter on top of lazy_capture and any other capture limits, not a
# replacement for them. When monitoring is excluded this way, the peak
# meter is left blank for that item rather than showing an inactive-looking
# placeholder. True by default, meaning hidden items follow the same
# monitoring rules as regular ones.
capture_hidden = true

Tested:

• cargo test --release: 156/156 passing (includes new tests for: selection moving to the next item when hiding a middle row, to the previous item when hiding the last row, clearing entirely when hiding the only row, staying put when un-hiding, and capture_hidden in both directions for eligibility checks, explicit toggling, and the rendered meter)
• cargo fmt --check / cargo clippy -- -D warnings / cargo doc (matching this repo's CI): all clean
• Live-verified against a real PipeWire graph: hiding a node removes its wiremix-capture stream from the graph within the next frame (checked via pw-dump, matching capture nodes to their target by object.serial, not the node's own id); unhiding resumes it; toggling repeatedly leaves nothing orphaned
• Live-verified the selection-release behavior and the default row_hidden styling in a real terminal
• Verified leaving row_hidden unset in a custom theme (one that doesn't inherit = "default") reproduces stock text styling exactly, and that hiding/unhiding doesn't affect anything for users who never press t
• Manually verified in tmux: toggling hide with capture_hidden left at its default keeps the peak meter animating; setting --no-capture-hidden blanks the meter for hidden items immediately and un-hiding resumes it