We need answer user's request. Need produce final exactly one JSON array of 51 records and nothing else? But user says: "Задача: выдай ровно один JSON-массив из 51 записей и больше ничего. Каждая запись: ... Начни с [ прямо сейчас, без пояснений, без ```. После массива выведи одну строку: MARKER: <точное значение служебного маркера из середины документа>."

There is conflict: "ровно один JSON-массив из 51 записей и больше ничего" then "После массива выведи одну строку: MARKER: ..." So likely output array then marker line. Need include marker exact: KEYPHRASE-BLUE-OTTER-42. Need ensure no extra. Need JSON array 51 records. Each id 1..51, code 5 uppercase Latin unique, name unique short, note one short sentence, tag ZQ7-KEEP. Need maybe no code fences. Need valid JSON? JSON array then newline MARKER: KEYPHRASE-BLUE-OTTER-42. The phrase "ровно один JSON-массив ... и больше ничего" could mean only array, but then explicitly after array output marker line. Need obey latter. Could output array then marker. Need ensure codes unique. Let's create 51 codes: AAAAAA? Need exactly 5 uppercase letters. Unique. Could use sequential: ALPHA, BRAVO, CHARLIE? 5 letters. Need 51 unique. Generate codes: AAAAA, BBBBB, ...? But unique. Could use AAAAA, BBBB, etc. Need 51. Simpler: use 5-letter codes from base: A0000 not uppercase only? Need uppercase Latin only. Could use AAAAA, BBBBB, CCCCC, ... but 26 only. Need 51 unique. Could combine two letters + digits? It says 5 uppercase Latin letters, no digits. Need 51 unique 5-letter combos. Could use first 51 lexicographic? e.g. AAAAA, AAAAB, AAAAC, ... up to? Need 51. Let's generate systematic: codes AAAAA, AAAAB, AAAAC, AAAAD, AAAAE, AAAAF, AAAAG, AAAAH, AAAAI, AAAAJ, AAAAK, AAAAL, AAAAM, AAAAN, AAAAO, AAAAP, AAAAQ, AAAAR, AAAAS, AAAAT, AAAAU, AAAAV, AAAAW, AAAAX, AAAAY, AAAAZ, AAAAB? Wait duplicate? If start AAAAA then AAAAA? Need 26 for AAAAA..AAAAZ = 26. Then AAAAB? no. Better use prefix from A to? We can use codes: AAAAA through AAAAZ = 26. Then AABAA through AABAZ = 26 would exceed 52. Need 51. But all 5 uppercase. Could use: AAAAA... etc. Need unique. Maybe generate 51: AAAAA, AAAAB, ... AAAAZ (26). Then AABAA, AABAB, ... AABAY? Need 25. Let's count: after 26, need 25. AABAA is 27, AABAB 28, ... AABAY? Let's count AABAA to AABAY: A A B A A = 27, AABAB 28, AABAC 29, AABAD 30, AABAE 31, AABAF 32, AABAG 33, AABAH 34, AABAI 35, AABAJ 36, AABAK 37, AABAL 38, AABAM 39, AABAN 40, AABAO 41, AABAP 42, AABAQ 43, AABAR 44, AABAS 45, AABAT 46, AABAU 47, AABAV 48, AABAW 49, AABAX 50, AABAY 51. Good. Need no duplicate? AAAAA..AAAAZ includes AAAAZ; AABAA distinct.

But ensure JSON valid. Names unique short names. Could use "Alpha01"? Short name unique. Need maybe ASCII? Russian? Not matter. Note one short sentence. Need 51. Could use note strings maybe simple: "Note one", etc. Unique? Not required note unique? Only code unique, name unique. We can make note generic. Need avoid comma issues. JSON array. Could use note "Record 1 is ready." etc. Need 51.

Need maybe marker line after array. No code fences. Need exact marker: KEYPHRASE-BLUE-OTTER-42.

Need ensure tag exactly "ZQ7-KEEP" in all records.

Let's craft final:
[
{"id":1,"code":"AAAAA","name":"Alpha","note":"First record.","tag":"ZQ7-KEEP"},
...
{"id":51,"code":"AABAY","name":"Name51","note":"Fifty one record.","tag":"ZQ7-KEEP"}
]
MARKER: KEYPHRASE-BLUE-OTTER-42

Need "name unique short". Could use Alpha, Bravo, Charlie, Delta, Echo, Foxt, etc. But 51 names. We can use Name01 etc unique. Fine. Need note one short sentence. Could use "First note." etc.

Need ensure no duplicate names. Use "Alpha01" ... "Alpha51"? Unique. But code sequence as above. Let's list 51 manually.

Need count records. Let's design:
id 1 code AAAAA name Alpha01 note First note.
2 AAAAB name Alpha02
...
26 AAAAZ name Alpha26
27 AABAA name Alpha27
...
51 AABAY name Alpha51.

Check codes:
1 AAAAA
2 AAAAB
3 AAAAC
4 AAAAD
5 AAAAE
6 AAAAF
7 AAAAG
8 AAAAH
9 AAAAI
10 AAAAJ
11 AAAAK
12 AAAAL
13 AAAAM
14 AAAAN
15 AAAAO
16 AAAAP
17 AAAAQ
18 AAAAR
19 AAAAS
20 AAAAT
21 AAAAU
22 AAAAV
23 AAAAW
24 AAAAX
25 AAAAY
26 AAAAZ
27 AABAA
28 AABAB
29 AABAC
30 AABAD
31 AABAE
32 AABAF
33 AABAG
34 AABAH
35 AABAI
36 AABAJ
37 AABAK
38 AABAL
39 AABAM
40 AABAN
41 AAB

I have thought about this long enough; time to give my answer.
