test(edit): drive oversize-line acceptance through booted kernel_edit gate

[rwrife]

Summary

• Completes the final acceptance gap from DEMO-05: Add minimal multiline source creation and editing inside SecureOS #769 (left open deliberately by feat(apps): line editor for in-guest source authoring (refs #769) #779): the oversize-input bullet is now driven through the booted kernel_edit gate instead of host-harness evidence only.
• run_qemu --test kernel_edit session 2 now appends a 70-char line after the reopen print: expects EDIT:ERR:LINE_LONG, ends append mode (drain-to-terminator), and the following p must show the buffer byte-unchanged (EDIT:PRINT:3, two new expected markers). The session then finishes the normal append/save/quit-clean flow.
• dev/building.txt documents the visible 63-byte line limit for users; the pinned /apps/dev/building.txt sha256 is repinned in tools/disk_image_apps_dev_sha.json.
• App source (user/apps/edit/main.c) is unchanged — the behavior already shipped in feat(apps): line editor for in-guest source authoring (refs #769) #779; this PR adds the booted evidence + docs.

Verification

• Script + heredoc parse-checked (bash -n, ast.parse, heredoc-unquoted exec of the rewritten kernel_edit block confirms exact step strings).
• Ad-hoc host harness (stubbed os_* bridge + scripted key queue replaying the exact new session-2 flow, 12/12 checks PASS): reopen-existing, print(3), 70-char append -> EDIT:ERR:LINE_LONG, second print still 3 byte-exact lines, exactly one total write (only the legitimate later save), append/save/quit proceed after the error.
• Strict freestanding compile of the unchanged app (-std=c11 -Wall -Wextra -Werror -ffreestanding -nostdinc) clean; lint.sh green; validate_disk_image_apps_dev_sha.py shows no drift on the repinned entry (remaining FAILs are the pre-existing local-artifact-missing vendor/tinycc: pin libtcc1.a runtime-helper source set + host drift gate (sibling of libc-deps.json, refs #408 #410) #548 class, not pins).
• Authoritative booted QEMU_PASS:kernel_edit remains the merged PR's CI (this host is aarch64 with no x86 QEMU).

Test Plan

• build-and-validate green, incl. booted kernel_edit gate with the new EDIT:ERR:LINE_LONG / EDIT:PRINT:3 markers

Closes #769