actionlog: compaction, and the attachment-* history lines nothing reads yet

Two pieces of internal/actionlog that #149 named and _plans/041_page-moved-detection.md deferred. Both are about the log as history rather than the log as a base, which is why they went together and why neither blocked #149: the base check reads only the last ok line for a file, and it works today.

Compaction

#149 promises a compaction setting rather than a truncation, because the history has uses beyond the base — #148's status view, and answering "why did this page change?" — so bounding growth must not mean throwing the history away.

The growth is real but slow: a line is ~180 bytes, so a 200-page tree published daily is ~13 MB of uncommitted local state a year. Nothing breaks at that size; Log.read is linear and runs once per root per run via actionlog.Cache. So this is eventually-real, not 1.0.0-real.

What it needs deciding:

• What the setting is and where it lives. markfluence.yaml's settings are unknown-key-fatal by design (internal/project/config.go), which cuts both ways: adding a key later is cheap, but a project file written for a newer markfluence is refused by an older binary. That is the established trade and needs no new answer, only a name.
• What compaction keeps. Keeping the last N lines per file is the obvious rule and is not the same as keeping the last N lines, since a batch writes one line per page. Keeping everything newer than a duration is the other shape.
• Rewriting a file whose appends are unsynchronized. Every writer today is a single O_APPEND write with no lock, deliberately (the project.SetPageEntry/client.SetContentProperty posture — a lock file brings stale-lock handling to a verb a person invokes by hand). Compaction is a read-modify-write over the whole file, which is a different animal: a concurrent append during the rewrite is lost. Renaming within .markfluence/ is safe from git's point of view (the planted .gitignore holds *), so the temp-file-and-rename shape is available.
• When it runs. Opportunistically on a write past a size threshold is the cheap answer; a markfluence verb for it is the explicit one.

One consequence already recorded in the plan: compaction adds another way to lose an entry, and that case is the same row as "no line naming this file" — unknown base, publish, silent. It needs no new semantics, which is what makes it safe to add after the fact.

attachment-* history lines

attachment-upload and attachment-download are #149's "history only" rows: they pair a local file with a page, so they could log, but nothing would read what they wrote.

They were deferred on a specific measurement rather than on effort: an attachment upload does not bump the page version, so an attachment write cannot invalidate a base and cannot participate in either check. A logged attachment-upload line would be a row no reader consults.

So this is machinery for a consumer to ask for, and the consumer is #148 — a status view that says "this page's diagram was replaced on Tuesday" wants exactly these rows. Filing it now so the reasoning survives; it should land with, or after, whatever reads it.

Deferred alongside these and not part of this issue: info reporting the base ("last published by markfluence at v44"), which is #148's business.

Start with internal/actionlog and internal/project/config.go, then read _plans/041_page-moved-detection.md and the related #148 and #149 issues. Resolve the compaction setting, retention rule, rewrite safety, and trigger, and decide when attachment-* rows should be written. Done means an agreed design and implementation that preserves the existing base semantics.