feat: add warnstrict mode for non-fatal strict violations (#24)

[Koan-Bot]

Summary

Adds a warnstrict import option that warns about unmocked file access instead of dying. This addresses #24.

use Test::MockFile qw< warnstrict >;

When warnstrict is active:

• is_strict_mode() returns true (all strict checks still run)
• is_warn_mode() returns true
• Violations produce carp warnings instead of confess fatals
• Execution continues, allowing discovery of all unmocked accesses at once

Useful for migrating existing test suites to strict mode incrementally.

Changes

• New STRICT_MODE_WARN flag (bitmask 8)
• import() detects warnstrict tag
• _strict_mode_violation() uses carp instead of confess when warn flag is set
• New is_warn_mode() helper function
• POD updated with usage examples
• New test file t/warnstrict.t (7 subtests)

Test plan

• CI passes across Perl matrix (5.14–5.40)
• warnstrict produces warnings for unmocked file access
• warnstrict does not die on unmocked access
• Mocked file access produces no warnings in warnstrict mode
• Multiple unmocked accesses each produce individual warnings
• is_strict_mode() and is_warn_mode() return correct values

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