Rename Statistics savings terminology to output-size reduction

[Loufe]

Purpose

Follow-up to #92. Rename the Statistics savings terminology so it describes the quantity actually measured: the signed difference between source and produced-file logical sizes.

The default Replace workflow normally retires the source after successful promotion, so this value generally corresponds to reclaimed logical size. It is not universally disk space saved when an original is retained, a hardlinked sibling keeps the source allocation alive, or filesystem allocation differs from logical file length.

Scope

• Rename user-visible conversion savings labels to output-size reduction terminology.
• Rename the cumulative conversion savings chart and tooltip consistently.
• Rename remux savings to remux size change or equivalent signed terminology.
• Rename the corresponding core and generated IPC fields directly; do not retain compatibility aliases.
• Update projection comments, frontend formatting, generated bindings, fixtures, and tests.

Non-goals

• Do not change the existing calculation: known input size minus known output size, with negative values when output grew.
• Do not add live disk-usage, allocated-block, hardlink, or free-space accounting.
• Do not change output replacement policy.

Acceptance criteria

• No user-visible Statistics text describes the metric as disk or storage space saved.
• Conversion totals and the cumulative series use the same output-size-reduction term.
• Remux uses signed size-change terminology.
• Missing-size coverage and all existing aggregation behavior remain unchanged.
• Repository search finds no obsolete savings field or UI terminology except historical documentation that is explicitly discussing V2.