Improve mod update check accuracy

[Al12rs]

Refactor mod update check

Reworks nxmUpdatesAvailable for clarity, performance, and correctness, and
fixes a long-standing bug in the install merge flag along the way.

Behavior changes

• Update check now resolves the installed file by recorded Nexus file_id
  first, falling back to filename if not present.
• updateAvailable() short-circuit now flags any obsolete status (OLD,
  REMOVED, ARCHIVED, ARCHIVED_HIDDEN), not just OLD/REMOVED.
• Obsolete files with no active successor will surface the latest
  downloadable inactive successor (skipping REMOVED) instead of falling
  back to a separate mod-info call.
• The requestModInfo follow-up is now only triggered when the mod has no
  identifiable installation file at all (manually-assigned mod_id).
• setLastNexusUpdate now fires for every mod processed.

Bug fix

• InstallationManager::doInstall had a dead bool merge = false that was
  never wired to result.merged(), causing every install (including merge
  installs) to clear installedFiles on disk. Fixed.

Internal changes

• ModInfoRegular::m_InstalledFileIDs switched from std::set to
  std::vector so install order is preserved (most recent install last).
  Public installedFiles() API still returns std::set for to not break plugin API.
• New ModInfoRegular::nexusFileId() accessor returns the file_id of the
  most recent install.
• Update check uses filesById and updatesByOldId hash indexes replacing nested linear scans.
• Split up the long function out into some methods for redability.

Update check behaviour:

Scenario	Status badge	Tooltip "newest version"	Update indicator
Active file installed, no newer version in chain	none	matches installed (own version)	no
Active file, active successor in chain	none	successor's version	yes (via version)
Obsolete file (OLD/REMOVED/ARCHIVED) listed, active successor in chain	obsolete	successor's version	yes (via status + version)
Obsolete file listed, only inactive non-removed successors in chain	obsolete	latest downloadable successor's version	yes (via status + version)
Obsolete file listed, no usable successor (only REMOVED ones, or none)	obsolete	own version	yes (via status)
Installed file no longer in listing (archived & hidden by author) — successor in chain	hidden	successor's version	yes (via status + version)
Installed file no longer in listing — no chain to walk	hidden	unchanged from prior check	yes (via status)
Manually-created mod with mod_id assigned via edit interface (no install file)	unchanged	mod-page global version (via follow-up call)	yes if mod-page version > installed