fix(none.h): remove implementation-defined signed shifts in clip helpers (#211)

[deekshaNVIDIA]

Fixes #211

Problem

The four software-fallback clip helpers in Include/dsp/none.h
(clip_q63_to_q31, clip_q63_to_q15, clip_q31_to_q7, clip_q31_to_q15)
detect saturation by right-shifting a signed, possibly negative value, e.g.:

return ((q31_t) (x >> 32) != ((q31_t) x >> 31)) ?
  ((0x7FFFFFFF ^ ((q31_t) (x >> 63)))) : (q31_t) x;

Per C99 §6.5.7/5, right-shifting a negative signed integer yields an
implementation-defined result. These are exactly the lines flagged in #211
and by static analysers (e.g. MISRA).

Fix

Replace the shift-based detection with equivalent, fully-defined range checks
using the existing Q31_MAX/Q31_MIN/Q15_MAX/Q15_MIN/Q7_MAX/Q7_MIN
macros from arm_math_types.h. No signed right shifts remain; the only shift
left is an unsigned one in clip_q63_to_q15.

Behaviour is preserved exactly (this is a portability/UB cleanup, not a
functional change). Two quirks of the originals are deliberately kept verbatim
so nothing changes for existing callers:

• clip_q31_to_q7 saturates outside the signed 24-bit range (±2^23), not
  the Q7 range — preserved via the 0x007FFFFF / 0x00800000 thresholds.
• clip_q63_to_q15 returns bits [30:15] of the in-range value — preserved via
  (q15_t)((uint64_t)x >> 15), whose low 16 bits are identical to the old
  signed shift.

(If either quirk is actually an unintended bug, I'm happy to address it in a
separate PR — I kept this one strictly behaviour-preserving.)

Verification

• Bit-exact equivalence, real compiler (clang 22, C11): the verbatim
  originals vs. the rewrites over

  • all 2^32 int32 inputs for clip_q31_to_q7 and clip_q31_to_q15, and
  • 2×10^8 structured/random 64-bit inputs for clip_q63_to_q31 and
    clip_q63_to_q15.

  Result: 0 mismatches.

• The edited none.h compiles cleanly (no-DSP host config,
  -Wall -Wextra -Wconversion -Wsign-conversion -Wshadow, no warnings).

• Only clip_q63_to_q31 and clip_q31_to_q15 are called inside the library
  today (e.g. arm_recip_q31, arm_div_int64_to_int32, and several Q31 Source
  files); both are among the exhaustively/structurally verified functions.

Happy to add a dedicated unit test for these helpers if you'd like.