bug(mac): backspace does not follow rules

[mcdurdin]

With the KMN test result:

• bksp_kmn keyboard - with 18.0 or 19.0
• bksp_ldml keyboard - only with 19.0.193-alpha-test-15488 (from fix(core): handle backspace decomposition #15488)

For a compliant app, e.g. TextEdit, we can see that the output after typing is correct (VERIFY_CONTEXT tests), but pressing backspace gives consistently wrong results:

NOTE: bksp_kmn is giving the EXPECTED results for E_ACUTE and D_ACUTE given the .kmn keyboard has no decomposition rules

Test Name	Expected	LDML test result	KMN test result
TEST_COMPLIANT_E_ACUTE_VERIFY_CONTEXT	abc é	✅ abc é	✅ abc é
TEST_COMPLIANT_D_ACUTE_VERIFY_CONTEXT	abc d́	✅ abc d́	✅ abc d́
TEST_COMPLIANT_A_UMLAUT_VERIFY_CONTEXT	abc ä	✅ abc ä	✅ abc ä
TEST_COMPLIANT_E_ACUTE	abc e	✅ abc e	✅ abc
TEST_COMPLIANT_D_ACUTE	abc d	❌ abc	❌ abc
TEST_COMPLIANT_A_UMLAUT	abc a	✅ abc a	✅ abc

For a non-compliant app, e.g. Chrome, the same thing applies:

Test Name	Expected	LDML test result	KMN test result
TEST_NONCOMPLIANT_E_ACUTE_VERIFY_CONTEXT	abc é	✅ abc é	✅ abc é
TEST_NONCOMPLIANT_D_ACUTE_VERIFY_CONTEXT	abc d́	✅ abc d́	✅ abc d́
TEST_NONCOMPLIANT_A_UMLAUT_VERIFY_CONTEXT	abc ä	✅ abc ä	✅ abc ä
TEST_NONCOMPLIANT_E_ACUTE	abc e	❌ abca	✅ abc
TEST_NONCOMPLIANT_D_ACUTE	abc d	✅ abc d	✅ abc d
TEST_NONCOMPLIANT_A_UMLAUT	abc a	❌ abca	✅ abc

Keyman 19.0.193-alpha-test-15488

• Tested by @Meng-Heng on MacBook Intel macOS Sequoia / TextEdit v1.20 /Chrome v144.0.7559.110
• Also tested by me on identical setup

Originally posted by @Meng-Heng in #15488 (comment)

(Edited for clarity)