Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@dvd/dvd_dump.ixx`:
- Around line 1056-1057: The current condition calls
df.valid(nintendo::get_key(nintendo_key, l, df)) unconditionally, causing
expensive per-sector validation even when options.verbose is false; change the
branch to check options.verbose first (e.g., if (options.verbose &&
!df.valid(nintendo::get_key(nintendo_key, l, df))) ...) so df.valid and
nintendo::get_key are only invoked when verbose logging is enabled, preserving
existing LOG_R behavior.

In `@dvd/dvd_split.ixx`:
- Around line 191-199: The code calls df.descramble(key) before checking
validity, so invalid frames get altered even when options.leave_unchanged is
true; move the descramble call to occur only after validating the frame (i.e.,
call nintendo::get_key(...), then bool valid = df.valid(key) and if valid then
call df.descramble(key), otherwise honor options.leave_unchanged and possibly
zero the buffer), updating the logic around get_key, df.valid, df.descramble and
options.leave_unchanged to ensure descramble is conditional on valid.

In `@dvd/nintendo.ixx`:
- Around line 34-37: The current std::span(std::span(df.cpr_mai, df.cpr_mai +
8)) in the Nintendo key derivation is invoking pointer arithmetic past the
6-element array df.cpr_mai; change derive_key usage to build a safe 8-byte
buffer explicitly after df.descramble: allocate a local 8-byte temporary (e.g.,
uint8_t tmp[8]), copy the 6 bytes from df.cpr_mai into tmp[0..5] and then copy
the remaining 2 bytes from the adjacent df.main_data start into tmp[6..7], and
then call derive_key(std::span(tmp, 8)) (references: data_frame, df.descramble,
df.cpr_mai, df.main_data, derive_key, key_lba0).

---

Nitpick comments:
In `@dvd/dvd.ixx`:
- Around line 186-220: Remove the stale commented-out implementation of
descramble by deleting the entire commented block that contains the old bool
descramble(DataFrame &df, std::optional<uint8_t> nintendo_key) implementation;
keep the file focused on the active API (DataFrame::valid /
DataFrame::descramble) and ensure no remaining references to the old
free-function descramble or its helper logic remain in the file so the codebase
only uses the current DataFrame methods.

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@dvd/dvd_dump.ixx`:
- Around line 1047-1060: Inside the verbose-only block (guarded by
options.verbose) remove the redundant inner check by deleting the "&&
options.verbose" from the if that logs invalid frames; specifically update the
inner loop that checks if(!df.valid(nintendo::get_key(nintendo_key, l, df)) &&
options.verbose) to simply if(!df.valid(nintendo::get_key(nintendo_key, l, df)))
so the logging via LOG_R("[LBA: {}] invalid data frame", l) still occurs when
ctx.disc_type == DiscType::DVD, raw is true, and the data frame is invalid.