fix: ISBN-10 Generate outer mod-11 and restrict X to check-digit position

[binggao1230]

Summary

This PR fixes two bugs in the ISBN-10 implementation (isbn.go) and adds regression tests. Both bugs are latent — the existing test suite passes without covering them.

Internal-consistency smoking gun (no external oracle needed)

The library disagrees with itself on the sum % 11 == 0 boundary:

checkdigit.NewISBN10().Verify("0000000000")  // => true  (correct check digit is 0)
checkdigit.NewISBN10().Generate("000000000") // => 11    (out of range [0,10])

Verify confirms the correct check digit for seed "000000000" is 0, but Generate returns 11 — a value outside the valid check-digit range [0, 10] and not representable as a single character. Generate and Verify contradict each other within the same library, and the Generate → Verify round-trip cannot even be formed for this seed.

Bug 1 — Generate missing the outer % 11 (ISBN-10 check-digit formula)

Per ISO 2108, the ISBN-10 check digit is:

check_digit = (11 - (weighted_sum mod 11)) mod 11      // range 0-10, 10 => 'X'

isbn.go computed 11 - sum%11 without the outer mod 11. When the weighted sum is divisible by 11, this yields 11 instead of 0:

Seed (9 digits)	Weighted sum	sum%11	Before (buggy)	After (fixed)
"000000000"	0	0	11	0
"055555555"	220	0	11	0

The fix applies the outer % 11:

// before
return 11 - sum%11, nil
// after
return (11 - sum%11) % 11, nil

Only the sum%11==0 boundary changes (11 → 0); every other check digit (1–10) is unchanged. Regression guards: Generate("000000001")=9, Generate("000000006")=10 (the X case), Generate("123456789")=10 all remain correct.

Bug 2 — Verify accepts X in any position

Verify mapped n == 'X' → digit = 10 for any position. Per ISO 2108, X (value 10) is valid only as the check digit (the final character). Codes with X in a non-final position were falsely accepted when the weighted sum happened to be divisible by 11:

Input	X position	Before (buggy)	After (fixed)
"X026515627"	0 (first)	true	false
"0X00000009"	1 (second)	true	false

The fix rejects X in any non-final position. Valid ISBNs with X in the last position remain accepted (e.g. Verify("000000006X")=true).

Fix

isbn.go (minimal, style-matching):

1. Generate: return 11 - sum%11, nil → return (11 - sum%11) % 11, nil (+ ISO 2108 comment).
2. Verify: track the loop index and reject X in any non-final position (+ spec comment: X is valid only as the check digit).

The weighted-sum verification logic is otherwise untouched.

Tests

Adds isbn_edge_test.go (follows the existing isbn_test.go table-driven style):

• Generate sum%11==0 cases (000000000→0, 055555555→0) + range check.
• Generate non-zero regression guards (000000001→9, 000000006→10, 123456789→10).
• Verify X-in-wrong-position false-accepts (X026515627→false, 0X00000009→false).
• Verify valid-X-last + controls (000000006X→true, 0000000000→true, 0000000001→false).
• Generate → Verify round-trip property over seeds including the sum%11==0 boundary.

Verification

• go test ./... — PASS (existing + new tests).
• go vet ./... — clean.
• gofmt -l . — clean.
• Pre-fix: the RED cases above reproduced on pristine HEAD (5504ad35). Post-fix: all RED cases resolved, all controls unchanged, round-trip holds.