mod: Phase 1 - bring forward mod-exclusive files with ActivityManager migration

Brings ~55 mod-exclusive files to the upstream-based mod/master-resync branch:

Activities (migrated to new ActivityManager pattern):
- Clock/Time: SetTimeActivity, SetTimezoneOffsetActivity, NtpSyncActivity
- Dictionary: DictionaryDefinitionActivity, DictionarySuggestionsActivity,
  DictionaryWordSelectActivity, LookedUpWordsActivity
- Bookmark: EpubReaderBookmarkSelectionActivity
- Book management: BookManageMenuActivity, EndOfBookMenuActivity
- OPDS: OpdsServerListActivity, OpdsSettingsActivity
- Utility: DirectoryPickerActivity, NumericStepperActivity

Utilities (unchanged):
- BookManager, BookSettings, BookmarkStore, BootNtpSync
- Dictionary, LookupHistory, TimeSync, OpdsServerStore

Libraries: PlaceholderCover, TableData, ChapterXPathIndexer
Scripts: inject_mod_version, generate_book_icon, preview_placeholder_cover
Docs: KOReader sync XPath mapping

Migration changes:
- ActivityWithSubactivity -> Activity base class
- Callback constructors -> finish()/setResult() pattern
- enterNewActivity() -> startActivityForResult()
- Activity::RenderLock&& -> RenderLock&&

These files won't compile yet - they reference mod settings and I18n
strings that will be added in subsequent phases.

Made-with: Cursor

chat-summaries/2026-02-13_cleanup-summary.md

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+# Letterbox Fill: Hash-Based Block Dithering Fix & Cleanup
+
+**Date:** 2026-02-13
+
+## Task Description
+
+Resolved an e-ink display crosstalk issue where a specific book cover ("The World in a Grain") became completely washed out and invisible when using "Dithered" letterbox fill mode. The root cause was pixel-level Bayer dithering creating a high-frequency checkerboard pattern in the BW pass for gray values in the 171-254 range (level-2/level-3 boundary), which caused display crosstalk during HALF_REFRESH.
+
+## Solution
+
+Hash-based block dithering with 2x2 pixel blocks for gray values in the problematic BW-boundary range (171-254). Each 2x2 block gets a uniform level (2 or 3) determined by a spatial hash, with the proportion approximating the target gray. Standard Bayer dithering is used for all other gray ranges.
+
+## Changes Made
+
+### Modified Files
+- **`src/activities/boot_sleep/SleepActivity.cpp`** — Added `bayerCrossesBwBoundary()` and `hashBlockDither()` functions; `drawLetterboxFill()` uses hash-based block dithering for BW-boundary gray values, standard Bayer for everything else. Removed all debug instrumentation (H1-H20 logs, frame buffer checksums, edge histograms, rewind verification).
+- **`src/CrossPointSettings.h`** — Reordered letterbox fill enum to `DITHERED=0, SOLID=1, NONE=2` with `DITHERED` as default.
+- **`src/SettingsList.h`** — Updated settings UI labels to match new enum order.
+- **`lib/GfxRenderer/GfxRenderer.h`** — Removed `getRenderMode()` getter (was only needed by debug instrumentation).
+
+### Deleted Files
+- 16 debug log files from `.cursor/` directory
+
+## Key Findings
+
+- Single-pixel alternation (block=1) causes display crosstalk regardless of pattern regularity (Bayer or hash).
+- 2x2 minimum pixel runs are sufficient to avoid crosstalk on this e-ink display.
+- The irregular hash pattern is less visually noticeable than a regular Bayer grid at the same block size.
+- The issue only affects gray values 171-254 where Bayer produces a level-2/level-3 mix (the BW pass boundary).
+
+## Follow-up Items
+
+- None. Feature is stable and working for all tested book covers.