mg: E (new CAS) flag ignored when modifying an existing item #1301

Describe the bug
Meta Get (mg) documents the E(token) flag as "use token as new CAS value if item is modified"(doc/protocol.txt:558, in the metaget flag list). In practice E is applied only when mg autovivifies a new item (N flag); when mg modifies an existing item — e.g. touches its TTL with T — the E override is parsed and then silently dropped, and the item keeps its old CAS. Either the modify path is missing the override, or E should not be listed for mg in the docs.

To Reproduce
Against a default instance (memcached -p 11311 -l 127.0.0.1), over nc. use_cas is on (default). Sequence and actual responses:

set foo 0 0 3
bar
STORED
mg foo c t
HD c2 t-1 // baseline: CAS=2, no TTL
mg foo T90 E555 c t
HD c2 t90 // T applied (TTL→90) but CAS is STILL 2, not 555
mg foo c t
HD c2 t90 // confirms CAS unchanged

// contrast — E IS honored when N autovivifies a NEW key:
mg newkey N90 E555 c t
HD c555 t90 W // created; CAS set to 555 by E
mg newkey c t
HD c555 t90 Z // confirms CAS=555

Expected: because T90 modified foo, E555 should set its CAS to 555, so the second mg foo c t should report c555. Actual: CAS stays c2 on the modify path; E works only on autovivify-create.

Versions affected: reproduced on 1.6.44 (latest at time of writing). The relevant code (E parsed for all meta cmds, applied only on the mg autovivify do_item_link, with no case 'E' in the existing-item flag loop) appears unchanged for many prior releases, so likely long-standing — not an upgrade regression.

Root cause pointer (in case useful): E is parsed into of.cas_id_in at proto_parser.c:593; applied on autovivify via do_item_link(it, hv, of.has_cas_in ? of.cas_id_in : get_cas_id()); but the existing-item flag loop in process_mget handles T/N/R/s/t/c/f with no case 'E', so no ITEM_set_cas on the modify path. The parser comment at proto_parser.c:593 scopes E to // ms, md, ma, which suggests the doc listing E under metaget may be the error.

System Information

• OS/Distro: Ubuntu
• Version of OS/distro: 24.04.4 LTS (kernel 6.17.0-35-generic, x86_64)
• Version of memcached: 1.6.44
• Hardware detail: 13th Gen Intel Core i7-1360P (16 cores)

Detail (please include!)
No crash/segfault — addr2line/core dump N/A.
Fresh default instance. Relevant stats settings:
STAT cas_enabled yes
STAT maxbytes 67108864
STAT item_size_max 1048576
STAT num_threads 4
STAT binding_protocol auto-negotiate
STAT hashpower_init 0
STAT verbosity 0
stats (post-repro, otherwise idle): version 1.6.44, pointer_size 64, curr_items 2, total_items 2, cmd_set 1. Full stats / stats items / stats slabs are all defaults and available on request.