fix(flash): merge segments sharing a flash page

Found on the first real-hardware flash (PM3 Easy). The fullimage ELF has two
adjacent PT_LOAD segments (seg1 ends at 0x15E5AC, seg2 starts there) that share
the 0x15E400 flash page. build_blocks aligned each segment independently, so the
shared page was written twice — seg2's 0xFF-padded head, written last,
erase-programmed the page and wiped seg1's real bytes there (the SAM7 programs a
whole page atomically). That region holds version_information, so the flashed OS
booted but reported 'Missing/Invalid version information'.

Fix: merge segments whose block-aligned spans touch into one buffer, so every
shared page is written once with both segments' real data; 0xFF only fills gaps
and aligned outer edges. Re-flash verified on hardware: 3df2ed2e8 -> 303bd5873,
valid version, matches_pin true. Bootrom untouched throughout (fullimage-only).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_flash.py

@@ -105,6 +105,27 @@ def test_build_blocks_aligns_and_pads():
     assert blocks[1][1][-100:] == b"\xFF" * 100
 
 
+def test_build_blocks_merges_shared_page():
+    # Two adjacent segments sharing the flash page at 0x102200: seg1 ends at
+    # 0x1022AC, seg2 starts there. The shared page must keep seg1's real tail —
+    # writing it twice (seg2's 0xFF head last) would wipe it (the on-hardware bug).
+    seg1 = (0x102000, b"\x11" * 0x2AC)   # ends 0x1022AC (unaligned)
+    seg2 = (0x1022AC, b"\x22" * 0x100)   # ends 0x1023AC
+    blocks = dict(build_blocks([seg1, seg2]))
+    shared = blocks[0x102200]            # 0x102200..0x102400
+    assert shared[0x00:0xAC] == b"\x11" * 0xAC   # seg1 tail preserved, NOT 0xFF
+    assert shared[0xAC:0x100] == b"\x22" * 0x54  # seg2 head
+    # no duplicate block addresses (would mean a double-write of a page)
+    addrs = [a for a, _ in build_blocks([seg1, seg2])]
+    assert len(addrs) == len(set(addrs))
+
+
+def test_build_blocks_separate_when_far_apart():
+    # A real gap between segments -> separate groups, no giant fill between them.
+    blocks = build_blocks([(0x102000, b"\x11" * 16), (0x120000, b"\x22" * 16)])
+    assert {a for a, _ in blocks} == {0x102000, 0x120000}
+
+
 def test_build_blocks_pads_low_side():
     # unaligned start -> leading 0xFF padding down to the block boundary
     blocks = build_blocks([(0x102010, b"\x22" * 16)])